So just think about that, Charles. Draw yourself a picture either on paper or in your mind's eye:

1. Amp to DSP cable disconnected at the DSP only = (less or) no hum

2. Amp to DSP cable connected at the DSP and amp but DSP off = hum

What component (amp or DSP) therefore is the conduit of the hum? What is the source of the hum?

I'll admit it's tricky. Even for electrical engineers.

House of the Rising Sone
Out in the mid or far field
Dedicated mid-woofers are over-rated