Thanks for all your input. I do love the sound of this amp. I have used only tubes supplied by VTL and Bea Lam has been very helpful in this regard. However, I still have no answer as to why, other than the tubes were bad. Perhaps quarterly biasing will help (as ryriken has suggested).
It does have a triode/tetrode switch and really sounds better, in my system, in the triode mode. I am using a Melos MA-110 pre-amp, B & W Matrix I speakers, a Meridian 200 transport and 203 processor, A Rega P3 TT w/a Project Tubebox SE. I've recently added a Paradigm sub (but this has been added after I got the amp back from VTL, not during the problematic period).
It does have a triode/tetrode switch and really sounds better, in my system, in the triode mode. I am using a Melos MA-110 pre-amp, B & W Matrix I speakers, a Meridian 200 transport and 203 processor, A Rega P3 TT w/a Project Tubebox SE. I've recently added a Paradigm sub (but this has been added after I got the amp back from VTL, not during the problematic period).