Headphone Adaptor for Power Amplifier?

That seems like an extreme method to power headphones since you have to somehow absorb all that current…a few years ago I bought one of those little Chinese tube headphone amps (called a "Bravo") for something like 40 bucks including shipping and it's well built (alps pot, nice design) and sounds amazing (uses a spare line level output from my preamp which does not have a headphone out). I did put a better quality 12au7 in it but it didn't make much difference…still a great little thing.

I bought one of the Bravo amps myself and agree whole heartedly with Wolf. A remarkable little amp that uses 1 12AU7 I rolled mine it also, although the stock Shuguang isn't really that bad in this application. The price went up some but it is still cheap given the quality of the sonics. You have good headphones I am assuming.
The other path is to use a "vintage" integrated amp or receiver which use their regular amplifying circuits with resistors already built in for the standard phones out that was expected back in the day. I was told this over at Head-Fi but I still wonder if it really the regular amp circuit.
Another really popular small affordable amp that they seem to love is made by a company called "Garage 1217". The current favorite amp is called "Project Ember" they can be bought as kits or fully assembled. It has one tube and takes a wide variety of tubes- a tube rollers dream. However This is not a really inexpensive amp like the Bravo. The prices are listed.

It is called a Can Opener and is made by Flat Vinyl. I bought one last year. They are inexpensive and work.