Your Prochamber is going to quiet down any muffler you put on there, and by quieting down I mean deepening the sound. Also the Prostreest are meant to be used with cats to minimize any rasp. So you could try and swap an aftermarket catted X or H (with only two cats) and see how you like it. Of course that would be more expensive than replacing the mufflers alone...you could always look into 1 chamber flows or SLP resonators.
I'll look for this thread with a mean as hell Prostreets setup on an offroad X I heard a few weeks ago. I'll post the link as soon as I find it.
I dont think anything is louder than SLP "loudmouth" exhaust. However, it is NOT the deepest tone you can get. SLP is very raspy and retro car like, IMO.
Video of Full SLP exhuast and stage 2 cams in my sig...