A minor correction - a tuned-pipe/expansion chamber does little to "muffle" & in fact can sometimes add a resonant "ping" to the exhaust tone, depending on wall thickness - and lower cost ones are usually pretty thin, accenting this note (prolly from the reflected pressure wave).
Parallel duals won't decrease volume levels it's true, thay may actually increase exhaust noise as restriction may be reduced (usually the point of duals) - in line mufflers will decrease noise, but may negatively impact performance as the restriction/back pressure is increased, it may defeat a tuned pipe/expansion chamber's effectiveness as well as they're dependent on such... as they are dependent on header length, which may be something to consider as well. Bends in the system reduce noise, but can also effect performance.
A solution to help with chamber noise & for a miniscule performance gain (uniform temperature assists flow), you can use
High Temperature Header/Exhaust Wrap & while you don't need to coat it, I would recommended
HT Silicone Coating as it helps preserve the wrap.
Not an expansion chamber ofc, just an example of wrap;
The most effective option that doesn't negatively impact performance to any large degree is simply exhaust system length, here's an expansion chamber using the stock exhaust manifold, with the glasspack silencer modified to be nearly twice it's original length (2" OD aluminum conduit) - quite a difference with far better performance over stock - while the noise is only slightly less than stock, it's behind the rider and so much more comfortable to ride;
