Thanks for all the replies.

K. Tsai, I have tried running the following, with the positive from BNC to master and shield to ground, and again with these swapped just in case...

1.
AC200 -Master- DSP3200 -Slave- DSP3200 -Slave- SW1600

2.
AC200 -Master- DSP3200 -Slave- SW1600
AC200 -Slave- DSP3200

3.
AC200 -Master- DSP3200 -Slave- DSP3200
AC200 -Slave- SW1600

Non control the volume of the sub.

I'm stumped at this point.

No Darling, I've had it months!