im a tube junkie. i dont really listen to headphones much but after being to a headfi meet, i see the money making potential of building custom amps. im lucky that i can have absurdly large speakers in the house and my girl accepts it.
theres lots of schematics out there online. if you follow through with this let me know. i have bags of american 6sn7's including vt231's.
the sex amp is nice and simple but lacks gain. lots of good info on headwise and head-fi. im on head-fi myself but purely to sell tubes as i dont really listen to phones much.
the biggest issue in headphone amps is the wide array of impedances that have to be matched. you basically have to decide which end of the impedance spectrum you want to build for. nice thing is that since you are driving phones, you can do all triode and have enough gain. i'd look for a design that uses a triode driver into a triode output.
transformers are everything. spend the bulk of your money on your output transformers.
Good to know. I felt the transformer was a huge part of it but it's often downplayed by manufactures. High cost I would imagine. I think that I will study this week & try to place an order for some goodies this weekend. Should be fun to have another electronics project.