With a high efficiency cone driver, comes certain limitations, some of which are usually a high Fs or very limited motor. The Lowether appears to make a substantial compromise in the motor; but seems to keep a reasonable Fs. In addition, while I have never seen these drivers put under a proper distortion analysis vs. low frequency vs. spl; I believe it unlikely they offer anything special in regards to such. So, while I don't have specific data for this driver, I'll base the following on expected general rules that should apply, give or take 10 percent for error. It may be this 6" driver worked sufficiently for the church PA application down to 70Hz or so. Perhaps 55Hz, assuming the church shape you make mention by some chance acted as a large extension of the horn mouth, and assuming moderate SPLs are required in that church. Low SPL of course, has lesser requirements. With increasing SPL at lower frequencies, I expect distortion to raise rapidly, as well as for compression to start setting in rapidly. As for this being a 'hi-fi' execution, as you claimed earlier, one might question what should classify as 'hi-fi'. A large, highly resonant system with very limited dispersion and limited LF response is not my personal definition of 'hi-fi', but that's just my viewpoint. With this driver, to get what I would classify as marginal SPL vs. distortion LF down to say 40Hz range, I will presume would require a massive horn loaded design with effective horn loading re-enforcement down to that low frequency. From what I recollect, Lowther drivers have extremely limited excursion. Somewhere in the realm of 1-2mm, I believe. I consider 30Hz to be the earliest any hi-fi system should roll off. And dynamics at 30-40Hz should be able to reach a minimum of 95db at the listening position without audible levels of distortion to account for semi-sustained semi-loud/dynamic passages while playing music at least at medium-moderate average SPL; probably in the 80-85dB slow averaged SPL range.
-Chris