Jones Maxflow - Not great?

Dynomax ultraflo is the muffler I use and always get the same compliments...

"It sounds bad ***"
"I really like the way that thing sounds"
"Wow those ultras really made a difference, and the volume and tone is more bearable than a chambered at idle and cruise"
"Seriously I can feel the difference...this thing got faster"
You know... all stuff people rattle at you.lol

The thudding could be a shitty characteristic of the design. . Or the case could be a lil high and hitting the car under wot...
I always recommend pipes all the way to the bumper with tips 3 inches out past the bottom of the bumper...to avoid the smell being vacuumed back into the car.. of course a bad trunk seal will foil that if bad enough.