Let me recap:
Old amp and speakers: no problems for 25+ years.
Change to RF83s, amp fries a channel in minutes.
Change amps and go back to old speakers, no problems.
Hook up RF83s, amp fries a channel in about an hour.
Closely inspect speaker wires, look fine.
Buy new McIntosh amp, hook up to old speakers (with new speaker wires), no problems.
Replacement RF83s arrive, hook up to new Mac. Mac overheats at 25 watts indicated (RF83s hooked up to 8 ohm binding posts).
Hook up the RF83s to the 4 ohm binding posts, voila, works fine. Loud, nice to say the least.
Simple conclusion: these are NOT 8 ohm speakers, I have two cooked amps and a new Mac that say so.
Interconnects, speaker wires, etc are not to blame. The only variable was the RF83s.
Could two pairs be assembled incorrectly? I suppose its possible, but highly unlikely. They are just not 8 ohm speakers.