I purchased Flyin’ Miata’s Hush-O-Matic  muffler and electronic controller. Installation per their instructions was routine. The electronic control wiring on the bottom of the
underhood fuse box was a bit tricky. Small wires, tight space, new car… Ask for help from a competent friend if in doubt.  The results are outstanding! The muffler has a
quiet mode and an active mode. Quiet is only slightly louder than stock. A pleasure to drive and NO droning on the freeway. The active mode uses engine vacuum to
open a butterfly valve in the second tail pipe, allowing the exhaust to bypass the muffler’s internal baffles. The harder you press on the gas pedal, the more the butterfly
opens and the volume increases. It gives the car a deeper, throaty sound that really comes on without being too loud AND without being loud all the time. Flying’ Miata
has a video demonstrating the sound but it does not do justice to the real sound quality of mine installed, especially on a tight twisty road with lots of gear changes.    
After having various mufflers on 7 previous Miatas, This one sounds the best.  And at the push of a button, it gets nice and quiet for those times when nice and quiet is
Ken Johnson.  Link to FM website page shown below.