I've been toying with this idea. I love my Neco Soundlab V3. but its gain is tough to adjust and its not ideal for low impedance phones. On the other other hand the C421 isn't great for HD600/HD650, etc even on high gain.
It's a little ridiculous, but I've thought about strapping two amps to my ipod, maybe the C421 and Neco, or the C421 and E12, which as a special preorder I should be getting in a week to two.
Do any of you do something like that? Any pictures to share?