After a number of comments from my wife regarding the noise level of my two MP40wES's, I decided to just bite the bullet and purchase the QuietDrive upgrade modules.
The actual upgrade process is a piece of cake, and took all of 10 minutes to complete. The end result? Dead silent MP40's. I'm not exaggerating - you can't hear ANYTHING unless you put your head near the motor, and even then it's just a minor hum. The best test is to turn on NTM @ 100%. You'll have to actually look at the controllers to see if they're working - that's how quiet these are.
I only have one knock on this upgrade kit: now that my powerheads are dead silent, I can hear the overflow. D'oh!