Its the system set up, the 50 up to the mufflers is more controlled for sound. I haven't looked at the manifolds but the difference may start there. You delete the mufflers on a 45 and it's gonna get loud.....
I believe most of us that have the FX50 and s models want a better exhaust note but are afraid of drone as well. I am coming from an 07 jeep srt8 and had to experiment quite a lot to get the truck just right. I have a completely stock 09 FX50S and its WAY too quiet. If the muffler delete is best I am all in but please advise if their is interior drone. If none or little I will try first and advise all. If their is some I wanted to try the magnaflow muff and tip 14840 that is 2.5" in and a 4" round tip. That sounded awesome on my old trailblazer ss. Let me know on the drone please.