If I remember right my tires start to hum at around 40mph. Doesn't really bother me or maybe I've just gotten used to it. As soon as my budget allows I'll be getting a set of TKC80s, probably one size over. Those babies should really hum.
Although the tire tread is most likely the root cause of the vibration, it is quite possible that the howl itself is resonating from another component due to the harmonic vibration. Another words it might actually be the belt guards or maybe the rear fender that resonates the sound making it audible only at a certain vibration frequency. That might explain why people are noticing the howl at different speeds. No two machines are truly identical. If that component can be singled out, it would be possible to dampen the vibration thru insulated mounts. I had a car many years ago that had a similar issue and it ended up being the heat shield for the catalytic converter. It would sing out with a weird sound that made you think it was possessed by demons. Once I removed the heat shield the issue went away. So changing the tires might might stop the howl, but it might simply be due to the new tires having a different vibration frequency, thus eliminating the howl. Or a new set of tires might simply howl under a different scenario. Faster, slower, higher revs, lower revs. Changing any variable will change the harmonic frequency.
It's definitely the tires howling, not only tread pattern but construction. Pretty sure the reason these tires were installed by the factory is due to the ability to withstand off road abuse, and cost.
Just like truck tires, there's going to always be a compromise with a dual purpose tire, finding one that is acceptable to the owner is pretty much a personal preference.
The ties from Bridgestone that come originally installed on the SCR950 howl at speeds above 50 mph and really get loud above 75 mph. It is a known issue with these tires, they were never designed for use on expressways or at speed. Also they do not wear well. I bought a leftover SCR950 in August of 2020 and now by 5500 miles they both need replaced. Looking to replace with regular tires, not scrambler tires again. Also the original Bridgestone's have a tendency to slip when leaned over on wet or slick roads at speed.