You just need a full signal from the factory unit or amp. The last 2018 Honda accord I've done they actually split the high, mids, and sub. I highly recommend a DSP that can sum the channels together to get a clean signal. We tried using line output converters (LC8) but it ended up sounding...
I would suggest switching out to compact style amps, for example Blam RA 75D amplifier or RA 501D. It depends on what sound level/quality you are trying to achieve and speaker setup.
My idea of what I would do is:
Helix 8 channel dsp/amp for speakers
Blam 501D mono amp for Sub
For your...