If you do end up stuck and the Helix V Eight does the L-R, let me know - I'm planning on, but still pre-purchase on the miniDSP 6x8. I'm flexible if it does what I want, and especially if the app works better in "landscape" than the miniDSP app does (planning on trying to get slightly slick with an Android box that uses my in-dash touchscreen). Maybe work out a joint deal on that and that Mass 12 if you do decide to let go of both.
But then again, I might find myself in the same boat as you. I'm sitting on small full-range 2's that would at least be a fun starting point for a 3-way experiment in the future.
And here's another thought - if you had to cut the sub loose and simply not run that through the DSP, that wouldn't really penalize you, unless that Zapco doesn't have a subsonic filter or a steep enough Xover for you. If so, $100 you could employ a miniDSP 2x4 just for that purpose.
I'll follow this thread closely, I have a set of Focal tweeters from a K2Power set that are my personal favorite tweeters, and I'd love to use them - but I'm still completely open on the rest. Using a DSP and going all-active would make those easier to integrate, and I'm also looking to find something that I could put in all four door spots that can serve dual-duty as kick-ass midbass drivers and midrange...
To be honest, I wish I had four of the K2Power 6.5 mids. And now that I think of it - those were actually 2 ohm. Maybe I should see if there's any used ones out there... not sure how the new ones compare but I loved the ones from 10 years ago.