You might also take a look at the Audio Concepts Titan. It too is a highly musical sub, with speaker level inputs. The ACI line is highly rated, though not as well known.
I have been looking at the ACI Titan XL too, though as you metion it is less well known. There are two reasons that I am leaning against the ACI.
The first, and most significant, is that the ACI uses Speaker Level Converters for speaker level connections. I am not an electrical engineer, but I am skeptical that these converters will not degrade the sound in some manner. In contrast the REL (and the MJ Acoustics) seem to be specifically designed to take speaker level inputs. If anyone can enlighten me on this, I would be happy to learn.
Also, it seems that if the ACI Speaker Level Converters did not degrade the sound in any way, one could use them for connecting any speaker level amp source to any line level sub. If that were true, I would look at the Onix ULW-10.
The second reason is that the Titan seems to be THE product to buy from ACI. But it is just too big for me and I just got rid of a "large" sub: the paradigm PW-2200. I move a lot and got tired of moving that 80 pound box around. The Force is smaller, but I wonder how it compares to the REL Strata III, which is the same size and the sub that I have heard great things about.