Just made an account with ug for this problem im about to post so any help would be much appreciated.

I recently bought a Vox AD60VT Valetronix 60W modelling amp off ebay about 2 months ago.
First few weeks were great no problems at all, then it started crackling and getting gradualy quieter untill i had to turn up the master and volume controls to full to even get a sound out of it.
Id leave it a day and would be ok again, then a few days down the line the same problem would happen.
Its been really annoying, my friend suggested it could be a valve problem but im not really sure how to tell if its the valve if i open up the amp and have a look.

Any suggestions?

Thanks alot.