Connecting the amps is easy, just connect the pre-outs of your first amp to the line in of the HK750. But the HK750 is a stereo amp, I dunno how you're supposed to hook that to your sub which takes one channel. Does it have a mono switch? Maybe press that and wire your sub to either one of the outputs?
Or if you're not afraid of blowing the amp, you could try wiring the red for both channels to the red on the sub and the black for both channels to the black on the sub. Theoretically this downmixes the channels and allows the two channel's amplifiers to drive the sub in parallel, increasing power, but according to wikipedia you're supposed to add a bit of resistance between the two channel outputs otherwise the amps would overheat...
Or you could do your previous thing and hook the sub to the red of one channel and the black of the other? I don't even want to think about what this means electrically but if it worked for you before...