Have a look at J&M speakers unfortunatley what we do not see are the 7.25 inch speakers hidden in the fairings (2 of them) powered by an amps and 4x6 used as fillers. In the back if they have a box it may just have a 8 or 10 inch sub. Size does matter when sound is open to the elements which is why 7.25 speakers are utilized .
The little 4x6 are just not capable of clearly reproducing sound and are restricted to the amount of air they can actually push.
I was impressed to see what gave bikes that sound you talked about but as always its big powerfull speakers that you would not think could fit.
Have a look at you tube and you will see the comparison between speaker size , power requirements to get them heard. No different than getting good sound in a convertible.

Just more power and large speakers is all that is needed.