headphones for tube system

Looks like your preamp has analogue outputs(hi & low?)...If your using rca interconnects you could go with balanced I/C between amp & pre and use pre out to a headphone amp.Some expensive headphone amps have balanced inputs should you care to spend that much $...
As for head amp and cans I suggest you spend time going through the equipment reviews on "head-fi.org"...Headphones are a lot like speakers,ie:you have cans with the highly detailed sound,the romantic sound and the studio sound...Also figure out a budget.Headphones can top out at $100,000.00+...Best value for $ IMO is the $250.00-$750.00 range with HiFIMan,Audeze and Sennheiser’s new HD660 leading the way...

I use inexpensive Grados (SR80e) that replaced my unfixable old SR60s…the 80s took a while to break in (too much treble initially) but after a week or so they became astonishingly good, and when I've considered replacing them with other more expensive models from other brands (tried several here and there) I just stay with my Grados. I have closed back phones for recording, but the Grados get my reference choice.

Stax.

Any update?

+1 for Grado RS2e