Things to consider:
Thick wall, small diameter tube emits less sound, hence large dia, thin wall emits more. Don't run larger tubing than necessary.
Quietest systems exit behind the rear bumper, aimed parallel, never down.
Baffled mufflers are far superior than flow thru (perforated glass packed), and they don't lose their sound attenuation over time.
Un-equal length reverse Y-designs do even more. If designed right, they can lower sound levels like a muffler with no actual muffler via sound cancellation effect.
You will see many of these features on OEM exhaust systems. The fact aftermarket systems never match the sound attenuation of factory systems is no accident. Companies like GM spend millions to develop exhaust systems that do not harm performance, yet are very quiet.
In reality, there is not a huge HP hit running the factory exhaust on any GM I've bought and tested, including the Dmaxes. When every tenth counts though, you trade noise for a little more power.