IMO a "Stealth" bike its one that just mumbles along at idle
Seems once any bike gets off idle its clearly audible, just the nature of the beast
On the old GasGas mufflers,
If you judiciously hole-saw a hole in your muffler you can, using a coat hanger, strip out all the old glass and repack it. wipe down the outside with solvent (Acetone, De-natured Alch, or my favorite Lacquer thinner) and put actual DUCT TAPE [The aluminum stuff used on ducts and chimneys] to cover the hole. Leave tons of overlap and roll it down hard. It's not Ideal but its cheap and easy.
The most effective thing I've found BY FAR
is to make sure the back box (muffler) is sealed tightly to the mid-box, and the mid-box to the header! High-heat silicone gasket material or dedicated exhaust silicone does the trick, a leaky junction in either will get the neighbors on the phone in a hurry.
Also you might take out your mid-box and weigh it. When they coke up Hard that *Tabuga-Tabuga-Tunga-Ungh-ung-Ungah uh uh uhh*
type noise Becomes a Crisp barking crackle of *WHOP-Pap-AH-WHOPA-WHOPAP-WHOP-PA-Pah-POP ah*
type noise. When I got my first GasGas the mid box weighed 13.2lbs, a week's worth of De-coking later she was down to a mean lean 11 (And I still don't think I got it all out!) but It's noticeably quieter.
I ride a lot of urban stuff, just tooling around town, and even with repacked boxes, airtight flow, and careful Idle adjustment we've never been able to make one "Library safe"
It's always been my understanding that the mid-box acts as a pseudo-expansion chamber/TunedPipe, so anything after that is just auxiliary suppression. Should that ring true...if you were deadly serious about it, you could just use a big muffler off an enduro/Dualsport bike to tame a trials machine?
Ha! those beastly sized cans on BMW's