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.
I'm using 2A3 SETs now so this 300B SET will offer more than twice the power. I will hopefully have a chance to spend some time with the amp to compare. It's a version of the Moth Audio Si2A3 amp that is being built by Craig Uthus with outboard power supply and wired for 300B's. There will only be a maximum of five of these so I'd like to try it. My speakers are 95db now but I'm looking into some single driver models that are a bit more efficient (Omega, Hammer Dynamics) or some vintage Klipsch's. I'm just curious how that tube does with headphones, since I'd never give up my amp if I lost any headphone performance.
mikeg has an Antique Sound Labs AQ-1005 DT powered by 300b's, and loves it.
Mike posted this thread after listening to it with R10's. I had a chance to hear K1k's, R10, L3000, RS1, and HD650's on Mike's setup, and was impressed. Very brief listening at a small gathering at Mike's place, so no deep comments from me.
Interestingly, Mike's main headphone rig is that 300b powered amp through an ASL UHC headphone adapter. Results are very nice, indeed.