GOING BY YOUR DESCRIPTION
That loose screw sound you hear matches the symptoms of a freyed coil. That's bad news if that's what it is.
Transient high votage surges,& high humidity levels.will frey a voice coil.

That popping you mentioned, if it is sourced from a characteristic of your system, keep it from reaching your speakers. Pops like that, especially of considerable power, really upps relative usage,most speakers go south much sooner.
Is that hummmmmm. is it ~60Hz? Probably.
Getting a hum from one channel would send my eyes to the connections. Were there other changes you may have made before you last shut down?
Presuming grounds are present and correct make sure static grounds' power cords polarity are correct. Grounds are easily missed. Check it twice.
Sometimes you have to trace those hums. (ballasts, flybacks(TV), electric motors in clocks ect...) That's no fun at all.
Agreed here on returning the whole thing right away for good.


OFF ON A TANGENT with K R K

I too, have had great difficulty with monitors, until I got some KRK monitors.
They're a good investment, worth every penny. I can't say either of those things about any other speaker.
I'm not much to commend gear, it looks dumb reading your words back a year later
'oh yeah man those close n play's are reelly happinin' (whatever itis)
With KRK it's different. That Co. and it's product, checks out. (not to be confused with KLH!) Huntington Beach, CA (now a sub. of Stanton Magnetics LLC Hollywood as of 8/00)

Hope you resolve your speaker problem. Upsetting I'm sure.
MORPH!

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