Shaker sound system


My MCA 2014 has the shaker sound system. It is plenty loud and has good bass, but it is missing the treble. Can anybody recommend speakers for the shaker stereo? I heard somewhere that I am stuck with what I have whereas if i had the standard sound system, this would be upgradeable. it almost makes me think that my speakers have built in amps or something that would render after market speakers less than desirable. I hope this is all a myth!