Check OTR http://www.off-the-road.de/en/XT-660R-X/Exhausts/ they have several versions, if you can affor it the Acrapovic is the one to go but I am not sure that it will be much quieter

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that is how i feel about my XTR
Mods so far:
Hyperpro front springs; Hyperpro rear shock spring; modified air box; LED tail light; LED indicators + new relay; YZ pegs; Lucas aluminum bar; brisk spark plug; snorkel removed; DNA stage 1; second hand DynoJet O2 mod; crash bars; front and rear steel braided brake lines; soft EBC brake pads; mitas 07 tyres, 2in1 Metal Mule exhaust, Blocked AIS, tall OEM screen, OTR bash plate