Be sure to troubleshoot everything in the line that may be causing the issue, i actually had the exact same problem (humming that removing RCA's did not resolve) and it ended up being a poor ground on the head unit...ran a longer wire against my better judgment direct to the battery, problem solved.
As for the GT50, that thing was rock freakin solid for me for 2 straight years, and i ran all 5 channels as hard as they could go. The only complaint I had about the amp is those cruddy plastic feet, so i had a buddy of mine mill up some aluminum replacements, slid em in there...worked like a champ
At any rate...i'm sure you've done some pretty extensive troubleshooting already, but in case you haven't, do!