A comment and a question here:
I have owned two different 05 Mustangs. Both with the Shaker 500. To my ears, the sound is better than any I've heard in any of the other 55 or so cars I've owned. Granted, when just listening to the radio it is not great...but when listening to good quality CDs, it sounds great to me.
Maybe there is a difference in quality of sound from year to year or sample to sample of the Shaker 500 ?
Maybe it is the young ears, who want tons of bass response ?
O.K., I'm a 65 year old geezer, yet I DO listen to alternative Rock stations, AND love Punk Rock, (whether or not it is a current trend).
Personally I like lots of treble, yet not so much bass that interior panels of the vehicle are going to vibrate. I find, with the original factory system I have, that often I have to turn down the bass level slightly from the full amount that is available for some CDs.
I'm happy with what I have, from the factory. And most of the time I'm more interested in hearing the engine and it's exhaust note, with the sound system turned off.
Currently my carpet is removed, and the base of the back seat is removed. This allows me to hear the exhaust, the ring and pinion sound, even the fuel pump....noises which are the real music to my ears..