Headphone connect direct to Amp

There are adaptors by Russound, Antique Sound Labs and others that will allow you to do this with headphones for about $100. This way, you can use any headphones.
The other option, if your pre-amp has enough output voltage, is to buy coax to 1/4" or 1/8" adaptors at radio shack so you can hook up your headphones directly to the pre-amp outputs and skip the amp altogether. Beware, some headphones are much easier to drive than others.

Antique Sound labs makes a box for that purpose. Check the web site. Has a switch to direct speaker output to headphone or speakers. Sells for about 150. Just picked up a similar type unit from a fellow AGOer's by Russound, which is no longer in production. Far too many pre amps and amps no longer have a headphone output. Some integrated amps have not even a tape output or aux out in which you could employ the use of a headphone amp.

AKG K1000s hooked up to a Cary 300b or 2A3 integrated is a combo made in heaven. They are designed to run off the speaker posts, but with a custom adapter can run off the 1/4" jack.

Several models of Stax electret and electrostatic headphones connected to the speaker posts on the amp. I don't know if Stax headphones are still being made, but I have a 20-year old pair of Stax electrostatics that I still use and enjoy very much.

Some of the best Headphones out there are STAX. I have an older pair of STAX SR-80 electrostat head ph. and the set up will not work unless pluged into the binding posts of an Amp. Very nice sounding headPhones but as I am moving to an active set up, my amps will no longer be running in full range. So they will find their way to a new owner whenever I get off my ass and list them.