Question reguarding the "Premier Pioneer Sound System" that comes with the 3LT package on the 2.2L's:

Is this system any good? Does it have a sub or just 4 speakers and 2 tweeters?
How much do you think it would cost for a shop to install a 8-10" Sub + amp but leave the HU and speakers stock? Has anyone come up with a good way of hiding the stock? Maybe under the rear seat??

 

If it's the same pioneer system that's available on all the Cobalts, then yes, it has a 10" sub and an amp that powers the 2 tweeters, 4 speakers and sub. The stock H/U won't give it much bump though, or any additonal subs for that matter. When I installed my system I tested the stock 10" with just an aftermarket H/U in, and it pounds significantly louder.

There's really no better way to hide the stock sub, and there's definitely no way you could fit it under the rear seats (unless you have some kind of master electrician/carpenter/mechanic, and a ton of cash to blow haha).
I think all in all you'd be better off adding a 10" sub and getting a new H/U, sub and amp.

If you're looking to get it done cheaply, I think your best bet is Best Buy instead of a small custom shop.