i just got a custom job. i had a free flowing muffler (flowmaster) and simply got a shop to put it on with 2.5 from downpipe back. i cut the cat. out and do not have a problem witht the sound. although i had 2 holes in my exhaust and was very use to a very, very loud car. the whole thing really did wonders for power and torque. i posted some pics on thread above.
Actually i found out this depends on the car, and the brand of exhaust.
Saab4life, used to have a basic dual tip muffler ( DTM) with 2.5" inlet off ebay, and it sounds AWESOME. Had a beautiful rumble, and didn't get loud until he gunned it. Even then it sounded similar to a modded BMW.
For some reason it was damaged, so he since bought a Magnaflow. He of course likes the magnaflow, but i think he said it was a little quieter.
If interested in the dtm option, check out flowmaster series 40 or 50 if you are looking for a louder, snarlier sound. A friend of mine does nothing but mods on cadillacs, hummers and the occasional saab. He always uses the magnaflow because his customers ask for a quieter exhaust that still stands out from the crowd. http://www.flowmastermufflers.com/cgi-bin/flowmaster/mufflers.html