I would not describe it as 'really quiet'
but the 'straight through' design
30 inch long
stainless steel
DynoMax UltraFlo
for RV use
is almost as quiet as factory stock
but has a lower, deeper tone.
Add a second
16 or 24 inch long UltraFlo
as a tip and the system is quieter yet,
but still has a deep tone.

The Walker 'QuietFlow' is as quiet as stock,
but is aluminized steel construction instead of stainless,
and my experience was that after 3 years it began to rattle inside perhaps because a baffle's spot welds came loose.