I've got the Magnaflow cat-back and really like the sound.

Gives a real "sports car" growl but there is some drone in the 1800-2200 rpm range. It's stupid easy to install and well made. However if you want a really low pitched tone the Flowmaster is probably the way to go. I had a V8 '97 Cougar w/ a pair of Flowmasters at the very end of the exhaust and it would set off car alarms whenever i drove through the parking garage. Really deep rumble at idle...which sounds like what you are after!