OK, first - there is nothing wrong with your amp, and no- this is not just a problem with Vox Valvetronix amps generally!
Chances are you've probably just come up against the 'Fletcher-Munson' effect. FM affects ALL amps, but pure DSP (digital signal processing) amps suffer worst because (unless you have a top-end one such as an H&K Zentera) the processing power and software can't make the amp behave in the way that an all-valve amp does when its cranked.
As you crank an all-valve amp the valves get hotter, & the amps tonal characteristics naturally change such that the effects of FM are minimalized. That's why all-valve amps will often sound very punchy, can seem louder, & cut through the mix more easily than low-mid range pure DSP amps.
The good news is that because your Valvetronix has valves, and is not pure DSP, it will respond very similarly to an all-valve amp as it's cranked and it won't be as badly suseptable to FM as compared to say a Line 6 Spider or Flextone amp. However, because the valves are used quite differently in the Valvetronix as compared to an all-valve amp, you will still be hit by FM to some extent. So, what is all this about, and what can you do to make your amp sing at volume?
In a nutshell, your ears hear sound differently at volume and you must ALWAYS set your gigging patches at gigging volume, not room volume. All Valvetronix from the AD30VT upwards have a power attenuator switch. When you're setting patches for gigging, set this to maximum (or at least 30w - not a 1w or 15w practice setting!) The real trick is then to crank the amp as high as you can get away with at home (or even better, set your patches at band practice) and to raise the MIDS.
What you'll need to get your head and ears around is that great bedroom patches will mostly suck at gigging volumes, and great gigging patches will usually sound awful in your bedroom! That's why I have specially set gigging patches saved into one bank of my VC12! At gigging, you'll also need far less reverb, delay and gain than you think. One of the easiest traps to fall into is when setting your gain levels. To max the gain all you do is turn your gain control to maximum, yes?! (doh!). Well, in a word...NO Too much gain will simply muddy your sound (tube, DSP or hybrid). At gigging volumes, you need the sound to cut through the mix, and backing off on the gain & upping your mids is the way to do this.
GIOTG = 'Get it off the ground'. For gigging especially, it's vital that you get your amp off the ground. Ideally, it should be about 3 foot off the floor or at the very least angled up. You and the audience need to hear the right frequencies aimed at your ears and neither of you can do this if your amp/speaker cab is on the floor where sound is not only being projected at the wrong height, but you're losing volume and tone through the floor!
If you gig regularly, I would also highly recommend an external EQ unit to give you way more EQ control than you can get with just the on-board EQ of your Valvetronix. Trust me, it will be like taking off a blanket from your amp. And you don't need to spend a fortune. I use just a simple 7-band Boss GE7 EQ through the effects loop and it's amazing how it can lift your tone live.
The valvereactor system in the Blues actually reads the changes in the real speaker impedence. The chromes use a simpler system that is cheaper to make but less effective in that it only simulates the changes in a dummy speaker circuit.
This may not sound like a big difference, but it's quite significant. The chromes only started life as practice amps (AD15VT & AD30VT), & therefore Fletcher Munson' was never a consideration in its design. When the 50w & 100w versions followed later on (an after thought spurred on by the success of the 15/30's) they had more power, but the design was the same. What this means in plain English is that the chromes are more susceptable to FM than the 'Blue' Valvetronix amps - with the blues having the more efficient system because they were specifically designed as gigging amps.
There's a good explanation of the Fletcher Munson phenomena on Line 6 support, and Vettaville has this too. I've duplicated it here for you (but without the graphs - you'll need to click the link at the end if you desperately want to see these! ): -
My POD or amp sounds great at home, but sounds boomy and harsh on a gig. What's wrong?
How To Get Loud
Have you ever had this happen to you? You've spent the afternoon getting all your sounds perfectly tweaked for tonight's gig, but when you get there and start playing, everything sounds really..... not right? Things sound overly bright, but also a little 'woofy', so you have to fix things on the fly as the night goes along and silently curse your amp. The next day, when you set things back up at home, you go back to re-tweak your sounds, and suddenly they sound okay again. Are you going nuts? Have your ears suddenly lost it? Is there a problem with your amp? Don't worry, they're both fine; you've just been bitten by the Fletcher-Munson curves.
"What's this?", you ask. "I thought Thurman Munson was a catcher for the Yankees, not a pitcher, (although he hit the curve pretty well) and who the heck is this Fletcher guy?" Well, aside from the fact that the baseball trivia part of your brain is functioning just fine, there's a whole other story going on here. Although it may look a little daunting (especially that graph you see looming below), it's really pretty simple, so just bear with us a moment for the inside poop.
Fletcher and Munson were researchers at Bell Laboratories who demonstrated, in 1933, that the human ear (and brain) perceive different frequencies in a shifting manner dependent on level. Their measurements showed that your ear is most sensitive to frequencies in the range of 3-4kHz, and that frequencies above and below those points must be louder, in absolute terms, in order to be perceived as being of equal loudness. They also showed that the amount of increase of loudness in those other frequencies to achieve that perceived equality varies depending on what the overall SPL (Sound Pressure Level), or sound intensity, is in the first place. These discoveries helped kick off a whole new area of study called 'psychoacoustics' and brought you, among other things, that little button on your stereo labeled 'Loudness'. When they mapped our these curves (also known as 'Equal Loudness Contours') they looked something like this:
When you look at these curves, you'll notice that when the 3-4 kHz range is at 0dB (or just barely audible), frequencies at 20Hz (about as low as you can perceive a distinct tone) have to be raised over 60 dB (which is 64 times as loud. Remember that decibels are measured on a logarithmic scale, so this is also 1000 times the power) to be perceived as being the same volume. On the other hand, when the base level for our 'home' frequencies is raised to 80dB, the lowest frequencies only have to be raised 10dB (or be twice as loud) to be perceived as being the same volume.
Now what does this mean to you as a guitarist? Well, as we alluded to above, you'll notice that the curves flatten out substantially as you get louder. This means that the sounds you tweak up in your living room will have the low and high end boosted substantially (the infamous 'smile curve') to make those frequencies sound equally loud to the midrange frequencies to which you're most sensitive. When you take those sounds that you designed at around 60-70 dB (which is your basic living room, not gonna wake the neighbors or overly annoy the family level) and turn them up to the average 90dB+ stage levels, those same high and low frequencies will suddenly seem overly exaggerated making everything sound simultaneously painfully bright, yet woofy (kinda like a bad wine tasting description). Not only that, but those midrange frequencies (where the fundamental information about just which note you're playing live) are being overwhelmed by that, now excessive, high and low frequency information.
So what's a fella to do? Well, if you can manage it without driving everyone crazy, studies have determined that the optimum level for reference mixing (which would apply to sound design as well) is about 85dB. This is loud enough to start flattening out the curve, but not so loud as to seriously hurt yourself (unless you do it for 14 hours straight) Get yourself an inexpensive SPL meter, set it to 'A' weighting (which shoots for the equivalent of the human hearing sensitivity) crank up your amp so you're averaging 85dB, and tweak in your patches. Of course, 85dB is, to put it in easily understandable terms, 'pretty darn loud', so this isn't something you can do a 2 AM when you can't sleep 'cause you're worrying about sounding just right for the next gig. The next best thing is to schedule a rehearsal with the rest of your band where you can crank it up, and make your final tweaks while the rest of the guys are there cracking jokes about obsessive/compulsive guitarists. Your third option, and probably the easiest, is to study the curves above carefully, and remember that if your sound is a little mid-heavy and seems a little bit dull at living room level, it's probably going to be about right when you crank that sucker up live. Here's a potential approach. Next time you're tweaking up a tone or two, make two versions; one that sounds right at living room levels, and one that you think, using the stuff you've learned here, should sound about right at stage levels. When you play live, leave the first one alone, and tweak the second one (if necessary), then go back the next day and compare the two. Pay attention to how they differ from each other. Now try and make a couple more, using the same process. After you've done this a few times, you should be getting a pretty good feel for just what you'll have to do to get 'em right the first time. Presto, you're one step closer to that elusive Ph.D in Tone.
Now, if you're the type that really wants to dive in and get some serious information overload, you might want to try going here. This is one of the coolest online reference sites we've found in a long time, courtesy of Campanella Associates, an acoustic consulting firm. It's a fairly complete audio text and tutorial, that will give you more than you thought you needed to know (but not more than you should) about audio, acoustics, and sound.
And some more techy stuff here (with the graphs referred to above that I couldn't paste): -