OT: one of my monitors is possessed by the devil.

One of my KRK RP5's was emitting the loudest nastiest noise when I got in to the studio today. Pulled the signal cable and turned the volume all the way down and no change. So, I'm assuming it's toast. 

The quandry: do I replace just the one, or do I bite the bullet and replace both? They're great monitors...I've had them for a loooong time. I would probably replace them with the 2nd gen RP5 unless someone thinks that's a crazy idea.

No need to replace the whole unit, and definitely not a whole new pair. It's almost certainly the amplifier that's broken, and you can probably replace it yourself. Start by contacting KRK directly and find out if you can buy the amp from them, or if not, who's an authorized repair center. I'd think if nothing else you can probably order it through Guitar Center and hope for the best.

Of course, KRK may quote you a repair cost close to a new speaker, in which case I'd take that as a sign from above that it's time to upgrade.