Eris 3.5, cable from output on PC to the speaker. See (uhh, hear) how you like it.
I don't expect that the computer soundcard D/A is poor.
Yeah, try the monitors. Bet the sound is tasty. When I first put in my M40 pair I was really happy. Use the AUX-In jack from PC to monitor.
If the Eris monitors are anything like my AV40s you could plug the 44VSL outputs to the Eris monitors. As long as you don't do audio from both sources at once you are fine. I did this for a long time with the AV40. When I wanted the "real" interface I just pulled the 3.5 mm jack from the monitors. Never a glitch.