Audio Systems: VW Passat Monsoon stereo weak bass, vw passat, factory radio

Question
I have a 2005 VW Passat wagon with the factory installed Monsoon stereo.  The bass is weak on the left channel (both front and rear) and this is especially noticeable at high volumes in the lower frequencies (such as bass and drums), and it is the same whether playing the radio, CD's or tapes.  The dealer just replaced the amp but this seems to have made little or no difference.  Do you have any idea what the problem might be?

Answer
i seen this problem before  and replace the amplifier and made no diference  later a replace the radio with a pioneer and the problem was gome

so the problem is on the factory radio or the factory wiring the goes on the righ side of the vehicle near the bac seat
have them change the radio just to test for a few minutes and most likely that will solve the problem