Loud thump then whistle.. Speaker is dead

Have you switched speakers from left and right channels to verify that it is indeed a speaker and not an amp channel?

Timlub

Got some time today and did exactly what you suggested. It has to be the channel because the speaker worked... Good and bad, sucks cause just picked up this B&K avr307

Even though you are repairing your B&K, you really need to get an ohm meter and check the dc resistance on each speaker. They should be very close. I suspect you might have a partial short going on that caused the channel to go out, you don't want to get it repaired only to have it happen again