Not a flat 6, but I have these mufflers. They seem to do alright at 2k rpms. I still need to put the car on the ground and give it a good rev to know for sure.
As for quieting them down; an insertable motorcycle baffle might be the best way to give it a bit of "quiet". Thats what I'm gonna do if my V6 is too obnoxious during driving.
https://www.ebay.com/itm/2-X-4-Exhaust-Muffler-Silencer-Baffle-Insert-Motorcycle-Chopper-ATV-Db-Killer/153732248608?hash=item23cb27f420:g:~CgAAOSwJzpc0yP 3
^^^ couple of these might do it right.
