Neat. I have some 6AV5s lying around - when I build my next pre I may give it a try.Love my 12B4 parafeed pre, but finding non-microphinc 12B4s has been a real pain.
No problem with pentodes/tetrodes here. I run 6CA7's or EL34s single ended in my parafeed 6B4 amp when the material calls for it. They really perked up my disc of "Green Onions" on Saturday!
Working up a 6688 driver for my 300b amp.