Hello Ash. Welcome to AOS.
There are quite a few valve headphone amps about. I'd suggest asking for some opinions in the Artist's Palette section. I reckon a good few members will have used valve headphone amps.
Enjoy the forum,
Main system: MMs/ADCs/Deccas/Low output MC's/various SUT's/various arms/hefty Japanese DD TT and hefty Japanese BD TT and small British BD TT/Swiss Idler jobs. 4 CD players/2 jitter buster/2 DACs/Valve buffer. TVC, stepped attenuator or valve pre-amp/Class A SS power-amp or Class A EL34 valve monos or big Japanese (part Class A) integrated. Big and smaller dual concentric speakers and full range speakers, amongst others. And too much more to list!