What about running everything at unity gain until the headphone amplifier, and use it's passive attenuation?
(note: this is a slightly different recommendation than digger945, because I'm assuming it's possible that you might have a active preamp than can actually give the signal some gain (For example, my BAT VK-3i could do this, and unity gain was at around 65% or so on the volume display), but I really have no idea)
If you have a 1kHz test signal and a DMM, I think you could just adjust each component until the AC volt reading on the input is the same as the AC volt reading on the output.