I'm trying to do a similar thing: connect SPDIF or Analog output on my amp's RCA input to mic it. I'm also trying to find impedance specifications for Toneport UX2, because I want to safelly plug effect pedals (such as Boss OD) on the Line Inputs, at the device's rear
I had the same issue with my Toneport UX2. Maybe this... Both are RCA, but the left one is called BNC, and the right one is the so called RCA we know. I don't know if this is the solution. Besides, there are also BNC/RCA adaptors.