I took the back box of mine and it sounded awful. I've got a stainless manifold, powerflow sports cat and powerflow midbox with a standard 1.0 backbox. It's a bit blowy under load because I don't have a flexi, it's just spring loaded bolts. Sounds pretty good when the blows not there. I'll be fitting a new back box soon hoping it'll change the note a bit. It's all about the note and not the noise though. If you want noise take your manifold off.