What do I need to keep in mind when picking a headphone amp? I'm looking for something that allows separate mixes, and not just separate levels of a single stereo mix. Is headphone impedance important (I know it is in matching live speakers to amps, I figure the same physics apply to headphones)? I am confused by the dizzying variety of impedance levels, and the impact they have. For example, I see some headphones listed as 600 ohm, and others as 55 ohm. The Behringer HA4600 is cheap, but says don't go under 100 ohms per channel, which throws out all the lower-cost headphones. The Oz Audio Qmix HM6 lloks like it runs things at 60 ohm / channel, which lets me user the cheaper headphones, but it costs 2 to 3 times as much, so the cost seems to offset. Headphone- wise I was looking at the AKG K240S (55 ohm) or K240M (600 ohm) for the control, and AKG K55 for musicians (32 ohm). Now I'm not so sure. This may end up in use remotely for musicians on stage in a pseudo-live recording, and double duty for overdubs in the control room.