I'll add my 2 cents, and a lot of $$.
I went through, a long evolution, in the muffler department. I started with the magnaflow 2.5 kit for the 'stangs.
At 2000 rpm, I had an obnoxious drone. Considering that from 1700-2300 rpm was cruising speed, it was not long before I changed the mufflers.
I changed to 19" magnaflow mufflers. Quieter, but still droned and resonated at the same rpms.
After researching, I found that some people had added a tee'd resonator, to their tailpipes, to eliminate the drone.
I did that. The "science" is that the tube, is to be 50% of the tail pipe length.
It worked. My car sounded like a quiet luxury car. Way too quiet.
I tried to figure out a way that I could have the car be noisier when I want and quieter when I want. I picked up a pair of summit cutouts and fabricated them into the tailpipe tee's. It worked perfectly. Quiet so I could listen to the radio, and noisier when I shut out the resonator tubes.
Then off came the 19" magnaflows, and on went the 14" magnaflows again, along with the installed resonator tubes.
This was better, flicking an electric switch, let me choose what sound level I wanted, quiet or noisy.
Still droned though, when through the muffler.
The other 2 problems were; I was carrying around an extra 10-20 pounds of hardware under the car, and it was in the way of my rear brake cooling ducts.
Screw-it, I decided to bite the bullit and get a pair of patriot varaflow mufflers.
Pricey but I really love them, and wish I would have discovered them sooner. Essentially they are a stainless 2 1/2" glass pack, with an electric flapper valve in the middle. Closed, the sound goes through the mesh and fiberglass surround. Open, basically like a glass pack.
One advantage, over the electric resonator set up I had before, is that you can vary the flapper part-way, and have something between loud and quiet, if you want.
They only come in 2 1/2" though.
Oh yeah, they do not drone... even in the open, louder mode.