Soundlabs are reviewed as having very good realistic bass, so a sub should not be necessary.
However before I got the Soundlabs I was going to try out a Klipschorn bass unit that I managed to get hold of - my theory is that it would make the ideal sub
I agree with Johnk - stay away from bass bins - far away.
Gordon Holt used Soundlabs (very fast) for many years before switching to ATC speakers (fast with panel-like mids and more dynamics). An 0.1/15 ATC sub will probably integrate with panels as good as if not better than most anything else - only problem is they are $7K which is like twice what a good JL subwoofer costs. If budget constrains you then check out a JL F112 first and see how that sounds (it is great value for a tight/fast sub).
On another point - how would you stop the bass from a poweful sub (like a JL or ATC) from interfering with the panels - surely the giant panel surfaces will be influenced by the air vibrations in the room. What I mean is this - when I play music loud then I can feel all the surfaces vibrating as they respond to airborne LF vibrations. Obviously a woofer with a huge magnet will be tightly controlled and remain unaffected - this is unlikley to be th case for a huge surface area of a panel in a relatively weak force field.