I seriously doubt one screw being stripped would affect anything. The reason I say this is because on both of my RM 40's each of the passive radiators have one screw stripped and it's not affecting the sound at all. I can assure you I have alot of power and push them sometimes with very bass heavy material and the bass is the best I've ever heard on any system. Other people have also told me it's of the best they've heard.
One thing to make sure is that when you changed out drivers that the wires aren't any where near the back of the drivers. On another pair of speakers I had one of the wires were just barely able to reach the back of the woofer on strong bass notes and it was driving me nuts. Moved the wires an inch and all was well.
The Megawoofers are very robust. Drive them hard, assuming the amp isn't clipping, and they'll take it in stride. I don't know what it would take to damage one but it would take a bare minimum of several hundred watts under just the right circumstances. Maybe even a thousand watts or more, yes I'm serious. I know this as I've owned very high powered amps driven to very high levels, well over 100db at my listening position, and have not damaged one.
