I like the sebring "soundpipe" for a centre section - I guess any back box should work inc a standard one..... it's basically a stainless straight through from the cat back for £40 (ish). I've got one and a custom stainless backbox, I don't like the look of my backbox (backbox was on when I bought it and sports a 4" oval tailpipe trim) but it sounds great..... not overly loud either
I've heard a V6 with the Sebring centre and a sebring backbox and it didn't really sound much different to mine..... perhaps a little tighter but the sebring back box has a twin 2.5"/3" round tailpipe so it would !....
I've got a Sebring twin-pipe job and it's just about as loud (or as quiet) as the standard pipe. My centre-section (trapezoid box) is standard thouigh, which probably makes the biggest difference.
I'd have a look at the Sebring competition pipe if you want a fruitier sound. It's not "TUV Approved" (a.k.a road legal, I'd guess) but they're like rocking-horse poo. I had one on order for months before I gave up and settled for the twin-pipe job.