If you want to eliminate cab noise and drone and be able to hold a conversation with the windows down you need a chambered muffler to eliminate as much resonance as possible. I think the Hushpower or something are straight through chambered.
Ive been real lazy about getting it on, but I have a 4" chambered straight through design from Car Chemistry called the Equalizer. If I get off my ass I'll try to put it on this weekend and let you know. They customized the I/O size and positioning for me for free as well. Can get it ceramic coated if you want, but diesel exhaust aint that hot