Sub in place!
Discret and perfect fitment, couldn't be much better if it was custom made. The Helix M One 600W@1ohm Class D amplifier (a different one than I originally planned) is hidden under the floor right next to the original amp, keeping the 8" woofer in check.
It's small, so it's not a wall of sound, but tight and clean bass that supplements the HK and adds both punch and depth. I now have negative EQ (2 or 3 negative steps) at 100hz to give the HK speakers more breathing room, and fill up that void with the new sub. Works like a charm!
As an intentional added bonus, almost 100% of the luggage space is still available, so I can keep the sub in when the whole family is on trips with the trunk filled with bags and stuff.