I have owned both the BHK pre and the 300's, and can confirm that there is almost as much to be gained with the tube choice in the amps as the pre. I first switched to the Tungsram 7DJ8's that BHK liked in the pre, and thought them a definite improvement over the factory supplied tubes, with better extension, bass, tonality. But what really lit my fire with the amps were Amperex 6922 PQ white labels from the early '60's. These didn't sacrifice frequency extension while bringing a bit more harmonic development and richness in the upper bass and midrange, which is something that benefits my speakers/system/tastes. So, that might just be the right "horse" for a certain "course", and the bottom line is that you get to tailor your system sound some via tube choice. I definitely believe the factory tubes can be bettered in any system, though.
PS Audio Amp tube rolling
Its time to replace my PS Audio BHK 300 monoblock's tubes. Does changing tubes from what were the factory set to another brand of tubes have as much difference as changing tubes in say a PS audio BHK Preamp. I changed/upgraded tubes in my BHK preamp and it made a fantastic difference. Will the same hold true for the mono amps (more, less, same). If you have done this "rolling" in the mono's what tubes worked for you and what differences did you hear.