BOOyah! Progress! After many months of head-scratching and curse-muttering, I can finally see (hear?) some light at the end of the tunnel.
Last night, John Rutan of Audio Connection stopped by, and he worked methodically through my lack of bass issue. In the end, it looks like it might actually be the M5-HP crossovers themselves. He installed the X-2 temporary crossover and, using the 33K jumper setting, my bass was back! All of it! So, as per John, the next step is to send off my M5-HPs to Hanford for a thorough checkup. Perhaps, John suggested, the caps in the M5-HPs have failed. Since I purchased these M5-HPs used in 2012, I have no idea how old they actually are. There also is some confusion over the jumper settings for the M5-HPs when used with the 2W series subs, as the settings were apparently revised, most likely after my M5-HPs were produced. So, we shall see. I hope to ship the M5-HPs out tomorrow. But at least I got the system up and running again with the X-2 John was kind enough to lend me.