...another vote for Bassani. I've had their cat-back system on my '03 GT for 8 years now and just love the sound. Not obnoxious; nice throaty rumble...the way a Mustang ought to sound.
I have the Magnaflow OR X-Pipe with the Magnapacks. NORMAL driving they are not too loud, but any blip of the throttle and it screams at you. It has helped keep from getting speeding tickets because I don't like attracting unwanted attention. At WOT they sound like pure hell, but some people like that. Driving on the highway at 70mph there is very little drone. This is not my DD. I probably wouldn't recommend them with and off road h or x pipe. The Magnapacks do sound really good with converters installed though. Just a little quiet for my taist.
Exhaust configuratin has as much to do with sound, as muffler selection does. A long tube set up, with off-road H-Pipe is going to make a particular sound of mufflers sound completely different than say a short tube version with an X-pipe. Pipe length, diameter, etc will also affect sound as does adding or removing cats. Let's not also forget how engine displacemetn and intake configuration affects exhaust note as well. So picking one muffler and making blanketed claims based on the way it sounds on "your" car is really meaningless for the next guy, unless he's running the exact same combination.
Many here talk how the Magnaflow's are drone free. I had 3 (yes 3) Magnaflow muffers and 2-high flow cats on my last car and it droned along as bad as any chambered muffler set up I've run.
If you want a completely "drone free" set up, you should probably stick with stock, or keep the exhaust diameter small and keep the cats in place. I've here the Dynomax VT's and old Turbo's are about as close to quiet as you're ever going to come, but I have no practical experience with either. And again...configuration is going to be a deciding factor as well.
Im going to try some original 40's (since I can get tem for next to nothing) and see how I like it. If I dont like it, I'll sell the flows and MAC's and get stingers (and probably weld in some hi-flo converters).