Which amp and DSP did you go with. I'm using a mono amp for my sub already. But I am having an issue where the Helix goes into some sort of protect mode on shut down, about the same time that the main unit actually shuts off (so 30 seconds or so after the soft shut down).I added a 5 channel amp and DSP to replace the helix amp because I got tired of having the low bass filtered out from the subwoofer. The helix amp seems to have a high subsonic filter which cuts out the deep bass quite a bit Anyway, you can improve the sound if you are savvy with tuning. I'm not sure how many filters are available per channel on the helix, but my DSP allows 10 filters per channel and upto 20 filters if you get creative with the setup. I do not recommend tapping the sub signal from the helix because it is filtered too much; the helix sub is quite capable when fed a proper signal and more power.
The helix will not come out of this fail state unless ground is removed. I'm doing this with a time delay relay which works but is a hack. It means I'm waiting 30 seconds for sound when I start up the car which is fine. But I have to wait that same 30 seconds anytime the speakers are turned off while driving (volume all the way down, end of a playlist, or I just want to drive in silence). That 30 second wait means that if I'm driving without music playing I will also miss navigation directions. I would like to go with something small that will fit on my existing amp rack, as seen here. I could always cut and carpet another one, but I'm lazy. And already having all the speaker wires back there courtesy of the helix harness means adding a DSP and amp is easy.
I quite like idea of the Audio Control D-4.800. Thinking I could sum the front speaker level inputs to get full range. Then use the 4 channels to power/EQ the front stage. I don't really care about rear fill so I would probably leave the rear powered by the main unit and keep the fader to the front. I never have people in the back but if I do I can move the fader closer to center, and they can deal with un-EQ'ed sound. I would then assign the single line level output to sub duty, feeding my existing amp. Thoughts anyone, or other suggestions?