Ok, so seems Def Tech is doing a few Bose moves here. While the built in subs are powered, a full signal is required as the speaker has it’s own crossovers but is not saying what it is, much like Bose modules and their cubes. There is a separate LFE input. Again, like certain Bose modules. So, you will want to set all three of the speakers to FULL BAND as the center channel is made this way as well.
The Onkyo has two SUB pre outs but you need three. I’d get a Y adapter for one of the pre outs and use it for the FRONTS. I’d use the other pre out for the CENTER CHANNEL LFE. Again, set it to FULL BAND. It may not make sense for a small center channel but they built in a powered subwoofer and crossover.
While I normally wouldn’t recommend DOUBLE BASS, you are in a spot and have to turn it on. The Onkyo doesn’t know that the speakers have built in subs and crossovers and will cut the signal to LFE(SUB PRE OUT) when FRONT speakers are set to LARGE. But, you need them set to LARGE so the speaker can apply it’s own crossover. DOUBLE BASS now has to be ON to get LFE signal from SUB pre outs to LFE input on speakers.
It may seem confusing and not all receivers handle using FULL BAND and LFE signals together the same way and with those kinds of speakers. I only discovered how the 696 handles it all after many hours of playing with it.