In My Personal Humble Opinion, neither of those tubes would be suitable for headphone application. 300B would be overkill - both in terms of cost and power - and 12ax7 is more of a pre-amp tube rather than an amp.
Suggest you go with something like a 2a3 (still lot of power than required by a headphone). But any decent triode tube with low(er) plate impedance, operating in Class A would make a great headphone amp.
For a start just google for tube headphone amp and see what you get.
300B not suitable for OTL operation due to directly heated cathode.
Sure. but you dont have to go OTL. with any valve FTMOF. A good transformer (like the lundahl mentioned) should be a very good interface between your headphones and the fire w/o compromising sound. The weakest link would still probably be the headphones.
Here's one that I've been eyeing after building two Millett-Hybrids and being charmed by tubes. I simply love the idea of using "living" antiques that were made before I was born...(I apologize in advance if you've all seen PM's pages before):
It's got a 12au7 and two ECC99, and output transformers to boot! Note there are two schematics, one specifically for AKG 1000 and one for the rest of us. The whole page is an educational read. Watch the high voltages!!!
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