I've had my Vox AD15VT for about 15 months now. Recently I have been quite pissed off by how the tone evolves during use. I don't notice much of the problem on clean or lowgain channels (might be because of my untrained ears on these channels), but it is very obvious on the "metal" channels (uk modern, numetal, us higain), which I use the most.

The problem:
When I turn on my amp, things are just fine, the sound is clear and just the way I want it. However, after about 15 minutes of playing it starts sounding more and more compressed, which could make lead playing sound a bit nicer, but the clearness in my rythm playing suffers. This annoys me, but I can keep playing for a bit more. But, after reaching maybe the 30 minutes mark, the sound is so compressed and muddy (don't know if that would be the right word to describe it, I'm a bit of a newbie at describing sound) that I can't take it anymore, I either have to switch to a clean channel or take a break (usually I do the last one).

I think I've had this problem all the time I've had the amp, but I think it didn't begin quite as early back then - and my untrained ear probably didn't care much. I guess there's something that gets overheated in there, but from the outside it's all cold so I don't know.

What do you think is wrong? And would a repair be worth it or should I maybe put my money into a new amp, like a Vypyr, which would probably fit my playing styles better?

Tl;dr: My AD15VT sounds really compressed and awful on higain channels after <30 minute playtime, what should I do?
could it be overheating? the same used to happen to my friends amp. we'd be jamming, itd sound fine at first, then it'd get really honkey and mushy by about 45 minutes.

it wasn't just our ears.. we recorded it and did an A/B comparison. it was quite a bit different from cold to hot.
Grammar and spelling omitted as an exercise for the reader.
That's what I think aswell, guess I should open it tomorrow and take a look.