My subwoofer is in the front of my family room and the home theatre receiver in the back.
Throwing a/d and d/a conversion into the mix can't be good for sound quality,
20-30 feet of shielded coax shouldn't normally be a problem with line-level signals. I believe you, I'm just saying there might be something wrong with the wires... Maybe they are not shielded. You say there are two... You only need one for the sub, so have you tried them separately? If you've got AC hum it could be a ground loop, and (electrical) S/PDIF may or may-not help with a ground loop. (And, obviously you can't go optical.)
ran a new cable outside the wall, with a bit of dressing/channel mount to make it look nice?