I got a reply from Jason at TBike....
Hi Don,
Thanks for your message
"The DB killers are small baffles which slip into the end of the muffler and are retained by a simple grub screw DB
they cut the noise output by 3 to 4 DB and do not harm power output too much.
Many Thanks"
I think I may get a set of these - if for nothing else than to have something to replace the originals next time I have to repack them.
