Well, first off, I know the quality of FM modulator and FM modulated devices are rather lacking, so lets move on from that little point to trying to diagnose the problem at hand.

The problem I'm having at the moment is with my 12 disc JVC cd changer, and how recently, the quality is so awful. I had the cd changer in my car for a little over a year, the quality before was definitely better then how it is today. Basically the problem really came about after I hooked up my 12" infinity sub and an amplifier, the sound quality of the cd changer suddenly went down the toilet.

The entire cd changer unit is pretty straight forward, fm modulator plugs into the antenna and antenna into the head unit (using the antenna adapter required), then theres a 12v remote lead, 12v constant lead, and a ground connection, then the connection between the cd changer to modulator and also the connection for the controller. The only thing that can really affect the sound quality would have to be a crappy ground, or intereference in the FM frequency. The ground connection has always been the same, so I can't really see it being the problem, and I'm not too sure about the FM frequency and the affects it may have on the sound quality.

Also the FM modulator is directly behind the headunit, as it has always been.

Anyone care to take a stab at this?

 

Is your amplifier a Class D one? I've owned class D's myself and not had issues, but I have read that they could interfere with FM signals if not desinged correclty. I would assume that Infinity would not have overlooked this. If your amp is a Class D, try disconnecting power to the amp and see if the problem goes away. If not also recheck all of your connections dealing with the FM signal.