I've been noticing a strange buzz coming out of my left channel front speaker. It began about a week ago and though is fairly constant, it sometimes crackles and hisses a bit louder at times, all whether the volume is up or down. Also, upon power down, roughly 5-10 seconds after the system switches off a much louder steady buzz is now emitting from the speaker for about 5 seconds in duration, making me concerned that something has happened to the my 7125's left channel amp section. I plan on running the left channel through the Surround Back channel just to try and verify if it is indeed the Amp channel that's bad or possibly the speaker. Thoughts...

You got it right. Swap amp channels and see if the problem follows the amp or the speaker. Buzzing can be caused by a blown cap(s) in the amp or a blown speaker.

Thanks EE....just wanting to check my logic and make sure I'm on the right track! Being the holidays I'll get to it when I do...