I own an AD30VT, but I don't gig with it--strictly a home player. But here's a couple of comments based upon what I've picked up at a couple of excellent Valvetronix user sites, Valvetronix.net and Patchtronix.com:
That change in sound you're referring to (sounds great at lower volumes, not so great cranked up) can often be caused by something called the "Fletcher Munson effect", and is caused by how the human ear distinguishes different pitches at different volumes. There's a "regular" in the user forums at both of those sites I mentioned who goes by the ID "Voxman", and he's very knowledgeable about this stuff. I'm quite sure he can give you some good advice on how to adjust your amp's settings to give a better tone. That said, the AD30VT is a small amp of limited power, so you can't expect miracles. But hopefully, you can get a much better result than what you're hearing now.
As far as the lack of presets goes, that's just an innate limitation of the AD15/30/50/100VT series. If you want more presets--as well as a couple of more amp models and a lot more effects flexibility--then the AD60/120VTX combos or the AD60VTH head would be the way to go, though they're (unsurprisingly!) much more expensive. Voxman can tell you all about those, too. He gigs with a 120VTX combo along with the VC-12 foot controller and an additional Vox 2x12 speaker cab. Quite the rig....