I only temporarily connected them analog in order to discern which ports had the hum on them (because the ProFire doesn't have any hum). Normally I run the ProFire 2626 with two Tangos connected via optical.
Computer->Firewire->ProFire 2626->Tango In and Out Optical
when measuring, it was either:
Tango Out->ProFire In or ProFire Out->Tango In. (I also tried Tango loopback, but looks just like the output.)
The power input section is a combination of Voltage Regulators and Electrolytic capacitors. I'm pondering whether I should re-cap them and see if it helps. I'll try to post a picture soon (been working too much).
Edited by barleyguy - 12/16/12 at 3:02pm