Your problem is not going to be in the speaker swap, but the headunit. The Shaker 500 headunit sux - plain and simple, I installed temporariliy an aftermarket headunit just to see if the car would sound better - it sounds amazing with an aftermarket unit all the levels are right on and you can adjust accordingly.
I have to disagree. the problem is not in the head unit .... It is in the line conversion of the factory amplified signal. If you compare a crappy line-out convertor to an aftermarket Head unit, then i agree with you that the after market unit will have a better sound, however, with the right line convertor connected to the proper channels on the shaker the end result can be amazing, and with the ability to maintain my navi, phone, and sync, is well worth a couple hundred bucks to acheive.
typically in any factory system the order of inferior products is:
3) Signal / head unit
and I have always replaced my systems in that order.
2012 GT CS Ingot Silver Vert, Edelbrock E-force supercharger with CAI and 3.5 Pulley, Tremec T-56 Mag, Accufab 84.5, Centerforce DF, Billet Yoke aluminum driveshaft, FRPP 3.73 Rear, MRT Performance H-pipe, Bassani axle backs, JL Audio sound, Edelbrock strut tower brace, JPC adjustable UCA, UMI adjustable panhard bar, KW V3 coilovers, Steeda heavy duty front mounts, Steeda rear LCA's, 20" Steeda Spyders with Michelin Pilot Super Sports, Steeda Big Brake Kit ....... it just keeps going.