On fixed system such as here with the factory premium system, the center channel is going to be the dominant one in the L/C/R (left, center, right channels) as member BARBOOTS indicated. Follow his advise on testings procedures. Mostly likely even the dealers's service tech's will not diagnose this correctly.
Most of the surround sound system both aftermarket and oem do not allow you to adjust the center channel as compared to some of the better home theater surround sound processors. The aftermarket JBL MS-8 can even give you a dominant center channel until is calibrated correctly. The Alpine aftermarket DSP PXA-H800 allows flexibility to adjust the center channel mix. Also the Arc Audio P8 DSP in the advance mode allows you to set a mix percentage for the L/C/R.
Sorry to say, their is no solution or work around unless you go the aftermarket route. For example; The cheap way out for this in the aftermarket world would be going with JL Audio 600/6 and feeding the L/C/R high level factory amp outputs directly into the amp's low level inputs via an RCA conversion cable (approx. $100.00 for 5 cables) and using the amps gain controls to offset the center.
As many of us have said here in the forum, correcting the factory surround sound system is not cheap. If you going to do it, prepare spend money.
Best for now.
Laredo X, Trail Rated, QTii, Sele-Terrian, Quadra Lift, Off-Road Adventure Package ii, Nokian Rotiva AT, Tow, AFE CAI, Custom Sound System ( HAT speakers, JLAudio , Pioneer , JBL MS-8 & RF 3.sixty.3 -- see the writeup link below for more details)