take it from me (and dr.z too for that matter), get RID of the NFB
and wire up your finals in UL or triode operation. that's the first
step. then use a low impedance grid drive on your outputs (i.e.
cathode or source follower) direct coupled so that you can push
into grid current without farting. last step, wail away. revel as the
screaming power tubes beg for mercy.

The Tesla 6L6GCs are rock solid and sound like a cross
betweeb a KT66 and an EL34. I've run these tubes VERY HARD
and had no problems with them whatsoever....A GREAT 6L6GC,
and IMO probably the best ever made!