I know you said your ground is good but I had a similar problem and I thought my ground was fine also. But my friend persuaded me to try a new ground and bam! The amp worked great after that.
I'm looking got some more 15" american bass xd's. I already have one and i'm looking for 3 more. They do not have to be in perfect condition but, I do want them in working. So bring on the subs, my paypal is ready!!