"(1) LCR tweeters (and/or crossovers) were somehow defective and chose to die at the exact same moment. I deem this to be extraordinarily unlikely."

In my experience, tweeters can either be blown immediately, or fail in increments. What monkeyplasm may have experienced is the final straw. The point where they simply quit altogether.

In my test bench set up, I've had tweeters in my speakers sustain short bursts of a loud transients from time to time, in one channel or the other, while repairing faulty equipment. What I've noticed is the tweeters were becoming weaker with each of these bursts and producing less and less output. Tests confirmed this in comparison with identical tweeters. Eventually they failed completely. I initially didn't notice 'til they got a certain percentage weaker.

"(3) LCR speakers can't actually handle as much power as they are rated for. I think this is technically possible but not particularly likely as there would have been people squawking about it before now."

Agreed. (about the "squawking before now", part)

"(4) RR2150 fed something to the LCRs that they couldn't handle. This seems more likely considering the simultaneous and identical failure mode for both LCRs."

I'm going to have to side with rdgrimes' Rule #1 on this.

The RR2150 has an analog mechanical volume control. you will reach a clipping point somewhere on the dial. This point could come sooner or later, depending on the level of the input signal.

I don't know if the Luxman utilizes an absolute volume, or a relative volume circuit.
The RR2150 uses an absolute. If the Luxman uses a relative scale, it most likely would not reach a clipping point even at the max setting.