I think the D-max Ultraflows sound great but are not gonna qualify as "Very Quiet" On a big motor car with 11.5:1 and a big cam they don't knock the DB's down very much...If you look at an UF it's essentially a perforated resonator tube in a muffler case.

D-max Hemi Turbo's really are reasonably quiet but arguably more restrictive than Ultraflows on a big motor at high RPM. But Real world 95% of the time... not very much performance difference on a properly designed system with collector extentions and an X pipe. I run them on a 12:1 ~720hp 517 and they're quiet "enough" ....barely. I had considered Ultraflows for this motor but with the cam and overlap I might as well run open headers as far as knocking down the Decibels. On my old 600 hp 508 they would have been fine though.

Dynomax has a muffler that is a hybrid of a short Ultraflow with a spring loaded Flapper valve, that may be worth looking at.

Hooker Aero's sound really good and are low restriction...based on your request that might be one worth looking at.

Edelbrock Stainless mufflers that had a split flow -<>- (Like an Ultraflow but with a double Y in the case) are fairly quiet and very low restriction as well. I think they copied the design from the SLP Firehawk. Not sure if they still make them anymore though, but they're a nice alternative.

In many states they're is an 80 or 85 DB max at 100 feet before they can cite you for noise.