Docrobbi,
A pair of 12" Rythmiks will produce (near) state of the art performance data down to app 35hz @ 100db - anechoic. In my room, I get flat response to 30hz w/no EQ and 25hz with app 4db boost below 30hz. I filter to eliminate response below 25hz, because it's a music only system and I have (virtually) no music w/info below 25hz.
I can't measure THD, but I certainly don't hear any and there is very little travel from the drivers at my max listening levels so I imagine it's pretty low. In short, I believe that there's plenty of room for more clean output and/or deep bass extension in my set-up, I just don't have the program material/spl needs to ever access it. Your room, listening levels, program material etc may well differ, so YMMV.
Marty
PS HT.shack has the Rythmik test results under DIY subs. The model that they tested was evidently a kit that, according to Brian @ Rythmik, was a WIP predecesor whose performance was similar to, though not quite as good as, the current model.