sorry should say I got it with CPSII
hey there! I sold mine a while back, but yeah, I bought three amps and each one did the ground loop thumping. I used a totally new harness as well, still the same. Sometimes it would be ok, sometimes not. I never got to the bottom of it, sorry.
I see that you have a capcom impress with q sound amp like me, have you tested this amp with cps3? I got a terrible ground loop only with cps3 and cps1.5 boards.