Hi, I have a VW Passat with factory speakers, there is a tweeter and a 6.5'' speaker installed in each door. Some time ago I replaced the old head unit with a Kenwood KDC-BT500U. The sound quality remained about the same as before, a wide range of sounds could be heard. Recently I noticed the sound lost depth, it seems that the high frequencies now overpower the lows. When I put my ear next to any of the bigger (6.5'') speakers I can hear a faint / muffled sound coming out. I have tried changing the head unit's settings which had little effect on the sound. I assume the speakers are connected correctly because their connectors can be inserted in only one way. There is no visible damage on the speakers.
Could this be a head unit issue? Is it possible that all the speakers have gone bad at once?