I just built a FirstWatt F5 and the B1 preamp and started listening to it on Monday. I never thought my old receiver was that bad, but I am hearing a lot more detail on my 4 Pi speakers. It is sort of like hearing my 4 Pi speakers for the first time again. Maybe that is exaggerating a bit.
It is probably not likely to occur in the near future, but I was wondering if you have noticed audible differences with bi-amping when passive crossovers are placed in front of the amp. My understanding is that the amp will not have to fight the crossover components and can just play the music. If there is an advantage, I might consider building a second F5 or another Nelson Pass design. At this point, it is just curiosity. I definitely do not need the power of a second amp.