Regarding LFE+main (aka double bass, aka enhance bass) & setting speakers to large, my personal experience is that I usually get best results with speakers set to small (regardless of physical size) and LFE set to LFE (not LFE+main, double bass, enhance bass).
There are two potential problems with those modes:
1) Subs and mains have to be phase matched at all overlapping frequencies, or else you can get bad cancellation. With the small setting, you only need to match phase at the crossover frequency, which is much easier to pull off.
2) Setting mains to large defeats their high pass filter. This can consume a lot of power and extension that could be saved with a high pass. Even subs typically benefit from a high pass filter (usually built in to amp.)
1) More speakers producing bass means more potential output capability (assuming no cancellation.)
2) More locations below the Schroeder frequency may help with room modes (or it may hurt, if they are the wrong locations).
Actually, I have had better results with small/no LFE+main so consistently (measured and by ear), that I don't bother even trying large/LFE+main any more. YMMV. If I had 215RTs, I think I would give it a spin, but not for any other JTR models.
Last edited by rcohen; 12-16-2015 at 07:23 AM.