Like shoot4fun said, go to 300BLKTalk. They have a lot of info, but look at info pertaining to your particular setup.
The brand of powder is somewhat contingent with your setup. However, the go to subsonic powder for an AR, is AA1680 with either a 208gr. Amax or 220 SMK. If you are running a carbine length set up and want to run subs, don't waste your time with any powder except AA1680. Lots of sad stories about people trying to runs subs with other powders and have had cycling issues. Pistol length gas systems seem to tolerate other powders in running subs, like AA5744. Though AA1680 is much like H4350, in that it can be very difficult to find. Good luck!!