I'm torn between the JFB 4554 like you have, and the v-tube v413f. When I called JFB and explained what I wanted to do (deeper, slightly louder sound; dual in/dual out) and advised the two choices I was thinking of, he really didn't recommend a specific JFB, he just advised that the v-tube has no drone.
I've been listening to the few vids of the v-tube on Challengers and Chargers, and it doesn't sound bad, it just doesn't overly impress me. The few vids of the FB4554 sound better to me.
You've had the FB4554 for about a year I believe....what do you suggest?
BTW in this video a DI/DO muffler was used that supposedly is a knock-off of a Flowmaster 40 - do you think it is the JFB 4554 - does this sound like your beast? How does the one in the video compare to yours?