At one time I had a pair of Acoustat 3's. In the room they were in bass response was so good I never thought about adding a sub. The low frequency at the beginning of Zarathustra was easily heard and felt. Unfortunately the next room I put them in never produced decent bass from them no matter what I did.
E-Stat has it right about different dispersion characteristics (Di-Polar vs. Cone) causing a discontinuity in the sound. I've always missed that coherence when using a sub but I've got to have my "earthquake" sounds.
The new subs I built (transmission line from Pass Labs DIY site) seem to do better in this area. I wonder if it's because of the way speakers are mounted. They are at the bottom of tubes sitting on spiked 12"x12"x1" granite slabs. The back of the magnets rest directly on the slabs with some Blu-Tack to couple them. They are crossed over at 75Hz..
There is no enclosure around any part of the speakers. The tubes are mounted on 1/2" thick coupling rings that slide into the tubes 1/2". They are glued to the rings with epoxy. The speakers are bolted directly to the rings. That's the whole thing except for some polyfil stuffing! I don't have any children or pets so knocking them over is not an issue. When I get around to covering them maybe I'll use some drapery tie-backs to ensure against falling.
Anyone who is a DIY'r should look into this design. They play loud go deep (18Hz with Eq) and are clean. They are also inexpensive and incredibly easy to build.