True Headphone AMP?

The only headphone I know of that can be driven directly off an amplifier is the AKG K1000 which has been discontinued for many years. Very unconventional in appearance and design but still only requires 5-10 wpc to operate properly. They are spec'd at 104 dB SPL with 1000 milliwatt (1 watt) input. Remember, those drivers are still VERY close to your ears, and each halving of distance from a point source increases SPLs by 6 dB. So if your normal listening distance is 8', by the time you get to 3" you are at a 30 dB increase in volume for the same output from the amp, assuming of course equal efficiency. Bottom line is with your Hafler you would be operating in the very non-linear portion of any attenuator's range, where channel tracking is typically poorest, distortion is highest and volume increases are logarithmic, and you would have an extremely difficult time adjusting the volume. So the K1000s work w low wattage tube amps, and due to their high impedance and unusual design, they require the most power of any headphone I know of. They do sound fantastic but given their replacement cost and difficulty in finding spare parts, I'd think very carefully before hooking them up to a high powered amp.

If you are still interested, here is the link to the ASL headphone junction box
http://www.divertech.com/asluhcsignature.html