I'm starting to get confused about your set up, you say you're connecting the profire and tango through an optical cable, so how are you measuring the analog outputs of the tango? Are you looping them into it's own inputs? Can I see a picture of your setup?
Here's my first thoughts and suggestions...
-How close are the transformers to the DAC's? Try moving them further away.
-Are you using shielded cables for your interconnects? Are the shields connected on both ends? If they are, try placing a ground lift on the output of the sources (output from the dac, output of the source feeding adc).
-Are these balanced or unbalanced connections?
-What is the case of gear made of?...
Last night I got to see Vomitor from Australia, killer band, currently on tour in the usa, go see them if you get the chance! I believe the lead guitarist and drummer are also in Portal, at least that's what someone told me last night, so there's that. Awesome classic death/thrash!