Long shot, but might also check head unit setup. Some JVC units, for example, can be configured to run a sub off of RCA's and mids and highs off of built in amp. As such, they have a high pass filter setting for the front speakers.

If you have a DVM, and can access the back of the radio, you can check resistance on the speaker leads...if you don't see the impedance of the speakers, you have a crossover or bass blocker somewhere.


Toby