The flowmaster 80 series is a good not too loud muffler.
I have the Dynomax Ultra flo Welded muffler on my 2.6 and I love it.
The best muffler out there as far as power increase is the power vault. It was originally designed for the VX but it's the same engine. They have dyno proven a hp increase of 20 hp. Now this does not come without some noise. Personally, I like the tone it puts out, and it sounds the best of all I've heard at idle and WOT.
If Kelli ever lets me do anything else to hers I'll throw a PV muffler on hers.
Joe