Pisa (I think) used to sell a non-muffler system. It had dual resonated tips, made by Hooker.
With that system, and a cat, it was obnoxiously loud. Droned like hell. I had a "bullet" glasspack added downstream of the cat and it was still pretty obnoxious, but tolerable. I could still drive through the parking garage at work, and leave a trail of wailing car alarms in my wake.
The Corvette "dual tips" that TLG were selling on Pennocks were resonated, too. I don't think they do much. Maybe just "take the edge off". I have one on my 4.9 car, with a cat and a glasspack upstream. With 2.5" pipe, it's still pretty loud. Probably not loud enough to get you pulled over, unless the cop wanted to be a pain, but it still has enough drone at highway speeds to be annoying. But then, I'm old...
Of course, that's with the 4.9. The characteristics of the V6 are much different. (The 4.9 was actually way too quiet with an Ocelot muffler, which sounds great on a V6, IMHO.)
Edit - At one time I had a single, straight through, chambered muffler on my V6 car. It sounded great, outside the car, but the drone was tremendous. It would plug your ears up.