I had all three setups on my car with high flow cats...H pipe, X pipe, and pro chamber. The pro chamber was much quieter at idle than the other two, otherwise- they were pretty much identical in volume.
I'm surprised it's that loud with stock cats honestly, but Flowmasters are just loud. Are you looking for less drone, less volume, or both? My current setup sounds great to me but I think it's walking the line between "acceptable volume" and "piss off your neighbors."
'05 Mustang GT Premium
CMS Stage 1 cams, C&L Racer, Lito tune, Jeg's oil sep, CMDP, SR UDP's, JBA stainless LT's & catted H, Magnaflow resonators, MBRP axle back, BMR springs, Koni STR.T, BMR LCA's & backets, J&M upper & mount, J&M Al panhard, DIY panhard brace, Steeda Tri-Ax, smoked corners, Raxiom Vector tails, Brembos, S550 GT Premium wheels, 2010+ door panels