This is what has become of the Gary Kaufman 6s4 amp I built a while back. It originally had a 6dj8 driver stage set up in an SRPP configuration. I'm not sure if it was the 6dj8 or the SRPP, but it always sounded a little edgy and harsh. Maybe there was some oscillation in there?

So one night I was feeling like soldering but didn't want to tackle one of the big projects I have half-completed, so I decided to stick another pair of 6s4s in there as drivers. I also soldered in a pair of NiMH batteries for the driver bias. The result is an improvement in smoothness and also in bass.