right, so I guess I should have clarified a bit, I took for granted that you were using a hi-low level input converter (a box that steps down voltage and turns speaker leads into RCA leads). If you are not using this, and just running the speaker level into a spliced RCA, then you are introducing much more input voltage to the amplifiers preamp than it was ever designed to handle...this is probably (one of) the source of the induced noise. Some amps have a high level input area that is designed to receive an already amplified signal, I am not familiar with that amp though.

This would also explain why you are able to have your gain down so low, your input signal is so hot it is disproportionate. Additionally, you will want your low pass crossover to be set around 80Hz, if it's down much lower you will be missing out on some music.

lastly, the +/- doesn't really matter as long as you're only using one. if you are using 2 (sets) then you just have to be consistent, if they are opposing then they will be out of phase with each other and you won't get much sound. The sub can be out of phase with the music and you will not be able to tell mostly due to variances in room acoustics.

Merry Christmas!