make sure that one of the speakers isn't hooked up backwards (out of phase). That would give it a "nasal" sound and would definitely hurt the bass/midbass aspects. Can also try sealing the doors (I'm assuming that is where the type-s speakers are installed) with sound deadening material (Hushmat, Dynamat, Raamat, etc...) that would help with the lower frequency extension as well as help eliminate some of the "tinny" noise from the sound wave vibrating the metal.
Or, it's possible that the speakers/deck is defective. Did it ever sound right and/or when did he first notice it sounded wrong? So many variables to consider.
