dmw,
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Read the specs carefully. The M80ti's enclosure is two inches taller and two inches deeper than the M60ti's. When you use the same two 6.5-inch woofers in a vented box of slightly greater volume, and you adjust the tuning of the ports and box, then you will get slightly deeper bass extension and somewhat greater output in dB at those frequencies.
Thanks to tleigh for referencing my article. A lot has changed since the 1980s in vented speaker design. But in a general way, you cannot get around the laws of physics. The enclosure volume and the woofer(s) total diameter and surface area determines both the depth and output of deep bass.
So to answer your first question: In a typical room, the M80ti has useful audible output to 25 Hz; the M60ti, to 27 Hz. Within their +/- 3-dB specifications (read my article to get a clear understanding of decibels and frequency response), the M80ti's bass output is -3 dB at 34 Hz; the M60 is -3 dB at 37 Hz. And so it goes: the M50ti, with a smaller enclosure gets down to 39 Hz (-3 dB); at 29 Hz, its output is -9 dB, about half as loud as at 50 Hz, although some gain can be expected from room boundaries.
Another fundamental (pun intended): Every tower speaker can benefit from a good subwoofer, which will extend and increase its output to the 20 Hz region or even deeper.
Regards,
