Hi, first of all, I hear NO sonic loss in P-P vs. SE. It is a different presentation, I would say, but the P-P topology gives the amp a lower output impedance and better control of the speakers as well as power. There is a bit of a romantic character with SET's that is perhaps replaced with a bit more controlled sound in PP.
However, when combining the beautiful tone of the 300B with 25W of real tube power, you get the best of both worlds. There is still a breathless and openness beautiful tonality AND bass response and control that only P-P can provide!
Most P-P designs use a little or A LOT of feedback. Most P-P pentode amps use A LOT of FB! This gives great bandwidth and lower distortion. It also detracts significantly from sonics in my experience! I considered adding a 1-2dB FB loop in this amp, but in the end I just loved the sound without FB too much! Our A30 mono blocks, parallel pentode EL34 amps also originally included a small amount of FB. During final testing and extensive listening one day, I cut the feedback wire and inserted a jumper, so that I could listen with and without FB. When FB was removed, the sound stage became HUGE and everything simply opened up! It was not subtle!
This amp achieves 20Hz-50Khz (-1dB) WITHOUT FB and also 25W of 300B power!
Even our SS amps use zero global feedback!
The 5687 was chosen for its unique qualities that are required to make this design work! It could not have been 6dj8, 6SN7 or 12AT7. Maybe 6H30, but I prefer the sound of the 5687.