If you are confident with the tools (or crazy), you could pop the lower drive unit out of each DSP5000 and attach a signal cable across each ones terminals.

Route the signal cables out of each DSP5000 (through the port so you don't have to drill any holes), wire each signal cable to the high level inputs of the DD-18.

The Sub will track the system volume as it's taking it's feed from the DSPs power amp.

This will work and is effectively free.

I would try it, but I have; 1, the training, 2, the skills and 3, a mind that comes up with 'different' solutions...

Good luck,

Steve.