it i slowly pull the L cable the sound gets nice and loud...but when they are both plugged in i get $hitty sound
oO
Well, it's less than preferable but a TRS/TS -> RCA adapter is probably the cheapest way to go. I'd suggest TS-RCA cables, but only if you have another use for them later.
+1 for your new interface, it's a good first step. While you're at Guitar Center check out the monitors and have a listen if you've got time. I'm not a big fan of GC and they'll likely push KRK's on you, but no better time to start shopping. If you are really serious about recording and mixing a good monitor setup is essential and will be 100x more productive than trying to translate your hi-fi setup.