DAC as Preamp for G.A.S. Son Of Ampzilla

So I recently was gifted some great equipment (G.A.S. Son of Ampzilla and Sumo Electra Preamp).
The Sumo, unfortunately, has about a 15min warm-up time where the left channel cuts out/has severe attenuation compared to right channel.

I'm also interested in a DAC/headphone amp to take Optical signal from my computer. Could I possible use this as a preamp for the Ampzilla? Are there perhaps issues with input/output voltage on the RCAs?
(NuForce Icon U-Dac2 is one I'm looking at)

Assuming I don't figure out what to replace/repair in the Preamp or shell out the money for a quality repair, is this solution viable?
Also, to further understand what my components are capable of, I've got a X-Fi Fatal1ty Soundcard that is currently feeding this setup. How good/bad is the DAC built into this as compared to a sub-$300 DAC/Headphone amp? The Front panel provides a 1/4" headphone output that I've been using now that I swapped my SR-3a Nakamichi for the S.O.A. and Electra.