There's nothing about a 300B that gives it an intrinsically "more interesting sound" than an 845. The differences are in execution more than the tube itself, though there are some characteristics that correspond to the power tube chosen. If you doubt this, get a 300B amp and then listen to Chinese solid plate, mesh plate, Sophia, Shuguang or TJ high-metallurgy, KR Audio, EAT, JJ and Russian 300B variants in it, and you'll hear that it's not the 300B that makes a 300B amp "interesting."
But look, they're both triodes, and applied single-ended, they will sound far more similar than either tube will to a push-pull implementation, or to a single-ended tetrode or pentode amp, using KT88 or EL34, respectively.
I have Audion Black Shadow 845 SET monoblocks on one system and Audion Golden Dream 300B PSET monoblocks on another. Sometimes I change which system each pair is in. The two amp types have the same power rating, but the 845 have more drive; the 300B have a little more finesse. But we're talking degrees. The 300B PSET amps have more silver in them (wiring + transformer secondaries). Both are solid state rectified. The input sections are similar; the driver tubes are the most different element.
I have had a number of very good 300B SET (not PSET) amps, and have had numerous 2a3, 45 and 6c33c SE amps come through my systems. The Audion 845, and I'll also say Sophia 845s had finesse and tonal density competitive with other makers' 300B and lower power SET amps. As well, most 300B amps have had too little discipline in the deep bass region to be usable on truly full-range speakers. I'll go further to say that to 70% of everyone who has heard both 300B and 845 amps on my main system, the 845 amps have the "more interesting sound," because of their dynamic energy and overall more vigorous drive.
There are many very fine 300B SET amps, including Audion's own, Coincident and Wavelength, but even on highly-efficient speakers, there is intrinsic value to the power and drive of a well-designed 845, for transient dynamic clarity that doesn't sacrifice agility, transparency, speed and finesse. Yes, equally well-designed 300B and 845 SET amps usually sound somewhat different from one another, but which one is "more interesting," will depend on what you value sonically, on a polar graph of imperfection.
If you have clarity, musical realism, speed and convincing tonal completeness from an 845, as well as same from a 300B SET running only 1/3 - 1/4 the power, the dynamic factors of having the 845's reserve and punch will show differentiating value even if you listen at moderate volumes. And to get the same power and clarity from 300B PSET will generally be more expensive to produce with all other factors in league or better. So don't discount a good 845 alternative on rumours of 300B SET being "more interesting."
My 300B PSET amps are among the 3-5 best power amplifiers I've ever heard at any price. They cost 50% more than my same-power 845 SET amps and they also better any 300B SET amp I've heard. My 845 amps are also in that class of 3-5 best amps so far. My 300B PSET amps couldn't be built and offered to the same retail price.
I like both; I have both. But if I could only have one, I'd have the 845 for being more versatile, more convincing in a wider variety of circumstances because of the 845's more robust dynamic behavior and it's better deep bass discipline. Other factors are fully competitive. In the context of having the 845 already, then the 300B PSET are appreciated for their more intricate finesse, though unmistakably at some comparative loss of drive.
Either tube can reside in a poorly designed and executed circuit that negates all of this.
Phil
