I do have Spectron Musician III Mk 2's with the Bybees and V-Caps. My speakers are separate panels and woofer modules. The lower mid driver does drop down to 3.3 ohm, but the speakers are not otherwise particularly challenging to drive. They do seem to present very favorably with higher level amplification. I originally used a Spectron channel for the panel and a channel for the module for each amp. Sound quality was much better than what I had experienced with ATI, Cinepro, and Gemstone amps on these speakers. I then tried the older legacy Wyred Class monos for the bass modules, and used the Spectrons in mono for the panels. Unfortunately, the woofers sounded muffled and congested. Lots of folks really like Wyred amps, and I wish it had worked for me, but did not. I have since gone fully active with the speakers--4-ways with Cary Cinema amps (Cinema 2 and Cinema 5)and the Spectrons. The gain is slightly higher on the Carys. I can adjust the gain on the woofer modules. I initially had planned on the Carys for the tweeter and woofer module, but the best sound has actually been the Spectrons on tweeter and mid, with Cary on lower mid and woofer. The upper range transparency, not etched, but smoothly detailed, extending into vocal range is very enjoyable.
I have not had the opportunity of auditioning the Cary 500MB's. I have read multiple high praise reports on them. I also have not auditioned the Bel Canto amps. Again, there seem to be many happy owners, especially on the newer models.
I was quite happy with the sound just using the Spectrons alone on the speakers, but curiosity and the quest was upon me. Sound quality is better now, but at considerably more expense and complexity. I'm also sure I will eventually do something different with the system, which may mean changes in amplification as well--so many options, so little time.
Will