I will echo a few things and then just give my opinion based on actual use.
if using a properly designed bullet, it will expand/fragment at sub velocities. I have proven that a white tail can be taken down with subsonic 300blk at less then 380ftlbs of energy and a non expanding bullet. I don't recommend it, but it can in fact be done.
while tumbling has been recorded to happen with non expanding subsonic 30 cal bullets in the 300 whisper, 300blk, in my testing, I have found it only to be consistent when using a slower twist barrel, ie. 1 in 10. my 1 in 7,1 in 8.5, and 1 in 8 while they will produce tumbling, continually do so at a point when it would have exited a game animal
if one is going to use a 300blk subsonic for hunting, you need to use one of the following in no particular order
lehigh defense 194 maximum expansion, (this has had reported lack of penetration issues)
lehigh defense 174gr controlled fracturing
outlaw state various copper jacketed lead core hollow points
cast hollow point ( my personal favorite)
anything else is not going to expand at sub velocities. as Recoil said, with the Amax, the best you can hope for consistently is that the tip will break off and it "might" tumble
as far as distance. it is going to have to be limited by the indiviudal shooter.
While i have no doubt my cast hollow point will do some serious damage at 400 yds. my range estimation is iffy at being precise enough at that distance to try it. 100 yds is good for me