Correct: I am running two interconnect cables the full length from the front sub to the back sub.

The Y-connector on the outputs of the front sub is most likely your problem (unless it's a stereo connect??). It's never a good idea to short/combine outputs together unless the manuals specificaly call out that they can be connected which is not the case for the Daytons. You can damage the output drivers of you amp.

I suggest you remove both Y-connects between the front and back subs and run a single cable Red-to-Red and see what you get. If that works run a single cable White-to-White. If that works too all is well with you amps. In that case running a single cable on the Red output of the front sub to a Y-connect on the input of the back sub will work if you don't want to run an extra set of wires.