True Headphone AMP?

It never really occurred to me but all the Headphone Amps I've seen are really headphone integrated amps, meaning with volume and sometimes source selection.

Has or does anyone make a true headphone amp that you simply attach to your preamps outputs?

Any reason this couldn't be done?

Given that I am heavily invested in my Pre and love it, seems kind of silly to use a headphone amp off the tape output that wouldn't use my pre amp for all the things it does right.
05-06-11: Almarg
If the amp is on a single chassis but is bridged or balanced, so that the black terminals are driven with a signal rather than grounded, you would do the same except that the ground wire would go to amp chassis.
Correction: The reference to the black terminals being driven with a signal applies to balanced amps, but may or may not apply to a bridged amp, depending on its design. In either case, with a bridged amp the signal wire would be connected to the output terminal that provides the non-inverting output signal, and the ground wire could go either to amp chassis as stated, or to a grounded output terminal.

-- Al
Thanks for the replies. I'm not sure I'm interested in a DIY connection to my main amps. It would be a pain to connect and risk of shorting when pluging in and out seems to much of a risk with 300W monos.

My prime interest would be connection to my Pre amp line out to an amp specifically designed to drive headphones.

I guess to narrow things down does anyone know of a head phone amp where you can bypass the volume and drive directly off my preamp?

I'll poke around more at head-fi, but first look found nothing.
If I were you I would still connect the headphone amp onto the tape(record) outs if you have such. I would not want my headphone amp depend on the preamp volume settings. If you don't want the signal go to the main speakers just use MUTE (once again if you have it). It will not mute the record out.
In DIY area to build a descent headphone amp is a matter of within $100 that would match your headphones AND your preamp perfectly.
In consumer market area not all of them may please you for the budget. The ultimate champ in budget-no-option is Grace M903 which is also an excellent preamp and 24/192kHz USB DAC. I've also heard lots of goodies about Violectric.
I've done this in the past to check for sound characteristics at low volumes. This was to see if what I was hearing was caused by the amp, or speakers. I would not want all of these amplification stages altering the sound in my headphones though, when a headphone amp doesn't need or use them. But for safety reasons, I would use a 1/4 to 1/2 amp fuse in each channel.

Others with more electronic knowledge might give a more appropriate value. This would be in case something happens to go wrong for example, an output transistor shorting and passing 50 volts or more to the headphones. Besides the loud volume, the phones can catch on fire (if not protected), with all of that power there.

If people reading this are considering doing this on a tube power amp, make sure you keep a reasonable load on the speaker outputs on amp at all times. I would guess a 8 ohm non- inductive capable of handling the output power from the amp.