Prodman,

That is an interesting dilemma!

The obvious test are:

Speakers:
1) Swapping dead speaker to alternate channel to see if works?
2) Swapping working speaker to dead channel to see if works?

Pre-amp:
1) Swapping dead pre out to alternate amp channel to see if works?
2) Swapping working pre out to dead amp channel to see if it works?

Have you tried all? Results?

As for the speakers, you mention the swapping of crossover . . . I build speakers as a hobby and unless there is a MAJOR design flaw with the speakers, or really really cheap parts, the crossovers should not blow. Possibly drivers will go from a clipping amp (most common cause). A simple 1st order crossover design might use 3 crossover components (inductor on woofer, capacitor / resistor on tweeter). A second order will use minimum 5 Components (inductor/capacitor on woofer, capacitor/inductor/ resistor on tweeter). As the design gets more complicated, the parts list goes up . . . usually a dead crossover component results in either the loss of the tweeter or mid woofer. If your speaker went completely dead, it is most likely a fuse they built into speaker cutting off all power. When you repaired the first speaker, did Monitor Audio confirm the fault? BTW: Is your center channel a different brand / Model?

The symptoms tend to lean towards a faulty amp. It is not likely to blow three speakers . . . unless you play so loud that the 7500 has hit its natural clipping point. If so you’re going to go deaf soon anyway and your speakers dying is probably a good thing (haha).