system sounds weird all of a sudden

I have a Cobalt orion 12" sub in a bandpass box, powered by a cobalt 150.2 amp. my head unit is a Kenwood KDC416.  I have had the system hooked up for nearly a year and a half and it has always sounded great but recently there has been some issues.  For instance, sometimes it sounds as if the sub and all my speakers are loud and clear, then for a few seconds they become muffled and distorted.  There is not a big differnece as most people who are in the car with me cant really tell, but I know what my stereo sounded like and something is messed up.  For troube shooting I checked the connections to the back of the head unti and things seemed fine, and I also checked my speaker polarities and those seemed fine too.  Any advice guys?

that's what it sounds like when you are a bout to pop............................................................an eardrum.you can put a bullet in my head, but you can't kill a word i've said

haha, i know, but unfortunately it doesnt have to be loud for me to notice it.  And today someone else who was in my car noticed it too

okay, ill check the ground for the amp and the ground for the head unit

Post your results. Also what kind of head unit do you have stock or aftermarket? If its stock do you have any high to low converters?

Im gonna check the grounds later tosay, but my head unit is a Kenwod kdc 416s im pretty sure. 

Also check for any loose connections. Check all your rca' power, grounds etc.