Mesa Mark 3 gone bad again

[sixstring63]

Hey Guys , Thought I had this Mark 3 thing all worked out but I guess I was mistaken. Recently I was playing and had a chance to crank it up good for the first time since doing the other repairs and it sounds good up till about 1.5 to 2 on the master and after that it gets bassy , flubby and mushy and the top end seems to roll off with no definition or tone that you can even work with, plus the volume does not really go up but just gets more muddy.I changed all the tubes but no go. I was using the amp at a lower volume for a few months and seemed to sound pretty good but sounds bad above 1.5 now. Any ideas ?

Thanks Mike