I'm a Live Audio engineer and bought these to use with some M20 K&M distance poles. They do a better job than my previous Adam Hall M20 sockets and were a closer fit.
I fitted these to a pair of Alto Black 18 subs, however, they didn't line-up perfectly. I'm not sure if this is due to the Alto's not being a standard fitting or the sockets not being quite right for a universal fitting. They needed slightly bigger holes drilling into the sockets (1mm) in order for the bolts to line up with the threads on the subs. They also could have had a larger diameter in order to spread the load more as they were smaller than the recessed area on the subs.
However, like I say, I'm not sure whether this is a problem with these sockets or with the Alto subs and so it would be unfair to mark them down without knowing for sure.
I may eventually secure them further to the M10 rigging points on the subs for more stability. But all in all... Another good product from K&M!