Albert-did you play with any subs when you had your soundlabs;just wondering as I have M2's.
Yes, had REL, Signet and Fosgate with Sound-Lab and the only way any worked were for LFE from home theater decoder.
It's very difficult to match subs with Sound-Lab because they are as close to perfect phase of any speaker ever made (in my opinion).
The big Sound-Lab may not even require subs. My U-1s went deep into the teens or 20 HZ region and with great weight and lots of air. If you crave more bass you could upgrade to big Sound-Lab for same or cheaper than cost of my two JL Audio Fathoms.
Just a thought.