I have access to a subwoofer for my theatre system (in the same room), but I am likely running this system as a strict two channel with my vintage amps. I'm thinking I don't want to add an extra sub to the mix, so am wondering if I should spend the extra on the speakers (HP version), get a mini-sub to get the lower bass, add a power amp for two channel (then I can't use my vintage receivers with the speakers, I think).

Tough decision. My 1970s sound consisted of some Sansui SPX-6700s (13" woofers, and a 4-way design), or later on a set of Bose 301s. The Sansui set were not 'great' speakers by any stretch, but were able to go loud. I think when Dad bought them they were $800/pair at Kelly's Stereo World in Canada. For 13" they weren't 'sub-woofer style' bass, but they were pretty decent for bass. Would the M60s compete, or would I want to go to the HP version?

WAF being considered, she may not let me have two sets of speakers in the Living Room, meaning my vintage amps go to my office with small bookshelves (M3s), and I end up putting the M60s into my AV Surround system with a two-channel Emotiva, and then they will have a subwoofer to augment the lows. Chances are I will have the two sets of speakers in that room for some time.