The TD15M was designed to be more of a midwoofer than the Td15h. M is for midrange. You can get it (15M)with the Apollo option but that isn't really necessary unless you plan to use it at high continuous volume. For home applications it isn't necessary and doesn't make a difference in SQ according to John Janowitz. I've never heard that the 15H has less distortion than the 15M when used as the designer intended. Its midrange is very flat and the accordion edge will pick up subtleties that heavier cones might miss. I like the higher efficiency 98db) so that it matches better with my compression driver.
My 1"CD goes down to 650hz. The 15M only has to cover ~ 700hz to ~ 70hz before the swarm of subs come to play. Low excursion means less movement which would seem to imply less distortion.
Exactly! I'd suspect the M has a nicer sounding midrange vs the H+ and I like the increased sensitivity too. While the M can play up higher it also has decent low frequency extension, overall similar to a typical 15" high eff pro driver like the JBL 2226.
The H+ does have much cleaner bass vs the M though, and the M a bit cleaner vs JBL 2226. The difference is mainly in the cone, the H+ has a heavier, stiffer cone with a larger diameter voice coil and larger motor. Subjectively, the difference comes through as the M sounding warmer and fatter, which tbh has some appeal over the cleaner H+ on some recordings and is closer to what most people are acclimated to. The H+ isn't as warm and has crisper transients, while also extending quite a bit lower. Subwoofer is optional with the H+ for sure, but with the M you're missing too much and subs are more of a requirement. With DSP especially, these speakers really don't need a subwoofer... the box is now tuned to ~22 Hz and with 19mm xmax I can EQ in a bit more low bass and still have lots of woofer to spare.
On the Apollo upgrade I couldn't get a straight answer on subjective improvements or measurements so I'm passing on that. It seems like it would be a good idea for heavy-duty/pro use but for home use in a woofer that's huge overkill to begin with I can't see it being worth the money.
The most important thing was to have the woofer integrate with the mids seamlessly. The sound character of the woofer, mid and tweeter are very well matched and I believe I have achieved a seamless driver integration with this design.