I have C4 tips on mine. The exhaust shop did a terrible job. For the harmonics, I don't recall being near anyone, but certainly no big trucks. If I speed up or slow down a few MPH, the sound goes away. It sounds almost like tire whine, but is metallic. Maybe some of the exhaust is rubbing against something.
For me, the Spintech is mellow, but has plenty of throatiness. I'm curious what Raydar thinks. I think he was the one that went through several mufflers on his 4.9.