Drone in my opinion is simply an acoustic oscillation that occurs at a frequency to where the exhaust pulse pressure wave travels down the pipe, hits the atmosphere (end of the tailpipe or another discontinuity) and reflects back. think of a guitar string. there will be a frequency where an exhaust system resonates with the total exhaust system length the biggest impact to the RPM where it occurs. the biggest problem is when that frequency of oscillation occurs at an RPM where we cruise (if it occured at 4000 RPM, then 99% of people would never notice). the muffler design can help attenuate the oscillation to reduce the amount of audible drone.
It's not just the exhaust length, the muffler has a large influence and the chassis design does too. The new Camaros have really bad interior resonance problems with aftermarket exhausts of various kinds due to the chassis. Fox3 Mustangs do as well, especially the hatchbacks, although not to the same extent.
IMO, Flowmasters earned the name "Dronemasters" because they have a lot of open volume inside that makes them act as a sort of Helmholtz resonator in the audible range. Toss in some constructive interference and you get very high amplitude waves in the band of frequencies we hear as drone.
One way to get rid of the drone that I have heard is quite effective is to weld closed-end resonators on each leg of the exhaust. They can be perpendicular or parallel as long as it is 1/4 wavelength of the target frequency. The target frequency is easy to find: simply get a clear audio recording when the exhaust is droning, open it with free audio software like Audacity, select a short section of audio, go to Analyze > Plot Spectrum, and there will be spikes on the graph at that/those frequencies.