First off, I'm glad to see there's still an active community for these speakers. I've had mine for 13 years now.
I've had 2 issues develop with mine recently. First was flappy noises from the sub, which I fixed with a foam surround replacement kit off ebay.
The next issue is strange. When I first start using my 4.1 promedias (I've been using mine as a 2.1 system), there's no bass coming out of the sub.
If I turn the bass all the way up from the control pod, then turn the volume up high, suddenly I have bass, and keep it even after turning the volume back down.
I'll keep bass up until I stop using them overnight. Whether I leave the speakers on or off doesn't make a difference, the issue is still there.