Someone may big dick me here, but the only way I know to lower the sub volume is to do so adjusting the gain on the amp/DSP, adjusting the bass on the head unit EQ, or changing the programming on the DSP.
Pretty sure you're right.
I'm looking forward to poly-fill stuffing my sub box and dropping the sub gain on the helix to bump the bass at the HU a notch or two. Because the EQ on the HU is only a 3 band, that bass setting will affect frequencies above the subs range (most likely, I don't have any specs to know for sure) so I'm hoping for a bit better blend and some more mid bass at the door speakers.
ATR if you haven't yet, check your Helix gain setting. If you're having to drop bass on the HU by 3-4 notches I bet it's turned up too high and you can get a better balance if you adjust that screw with your HU EQ set flat.