If we were talking twin cams with catalytic converters, I would advocate for entirely new exhaust. Since we are talking Evos here, a a good set of slip ons while keeping the same head pipes and crossover will give you the most bang for the buck and will sound good.

A large volume reverse cone megaphone muffler gives the best performance but usually looks like crap. If you want to get technical, the volume ideally would be ten times the volume of the cylinders it serves, which of course isn't practical or aesthetically pleasing. This information is from research at the Brookland's track long ago.

A large diameter long muffler will usually yield a deeper throaty tone while shorties tend to be raspier and in my opinion annoying sounding. Have a listen at Rhineharts and Big Louie's to hear what I mean about deep tone.
Granted, the above video is on a twin cam, but you get the idea as all of these systems are good slip ons with large diameters. In the video, I think RCXhausts sounds best but I have no experience with them. My dad has Rhineharts on his 08 Road King and they great, almost as good as my bike haha.

When I bought my bike it had no low end torque but looked stock. What I found was the previous owner busted out the all the baffles from the stock exhaust. At the time I was looking for a cheap and easy fix until I could decide on what to do. I ended up putting in Big City Thunder baffles which are essentially lolly pops. They worked so well and sounded so good that I never changed to anything else regarding anything to do with my exhaust. If I did do something else, it would be Rhinehart slip ons.

 

Unfortunately this video does not accurately capture my sound

 

Prot,

They do sound nice and I particularly like the deeper throatier sound.

 

Drop in a set of power cones and you'll get that low end right back, cheap enough.