Not personally familiar with that barrel, but a couple of articles I skimmed indicated that it was a decent barrel. What handguard are you using with your setup? Also missed any mention of an optic. Your listed components look good, and that ammo is supposed to be good. On your suppressor, you're not getting any baffle strikes, are you? How is your suppressor attached?
My 300BO pistol uses a BA 9.5" 1:7 barrel in nitride chromoly. I use the 208g Hornady ELD-M in my own reloads which is basically identical to the AMAX and in fact appears to be replacing the AMAX. I shot the 208g AMAX in Hornady's Black ammo first in a test with two other subsonic 200g+ rounds, and the AMAX was the clear winner. You have an appropriate barrel twist for the heavy subs, so I don't think that's the issue.
I use a SiCo Hybrid that is directly threaded onto the barrel...no shims or other components. At 50 yards off a rest I can occasionally get .75" groups of five but never worse than 1.25" groups unless I clearly had a flier I could attribute to myself. The 208g AMAX and ELD-M are very accurate if they're loaded right.
If your barrel is good my first thoughts go to the optic, handguard attachment, and suppressor attachment. Of course, I think I might buy some Hornady Black in that 208g AMAX to see how it compares for a quick check.