Haven't used 300B's with headphones, but use a couple of 300B monoblocs in my main speaker system. Most, but not all, 300B systems put out low power 5-10 watts so you need higher efficiency speakers, 95 db and above, to take full advantage of the amp. I've seen some 300B amps that put out 20-25 watts that will probably drive lower efficiency speakers. Don't know what kind of 300B setup you are talking about, but the pairing with speakers might be a limiting factor.
As far as sound goes, I think it is either like the sound or hate the sound. I like it. Probably best if you could listen to a setup before you buy.