On the rear deck speakers of my 97 A4, the speakers seem to be assymetrical. The left hand (If looking into the trunk) speaker has a metal case around it, labeled "2X20". Is it an amp? At first, it looked like a small subwoofer. The right hand speaker seems to be a normal speaker. Now, I'm wanting to replace the speakers in the car, and I'm not quite sure what I'm looking at. Perhaps some insight?