I have loaded both my 30-30 and 375 Winchester down to subsonic loads. The most accurate seems to be the 375 Win. with greasy 265gr lead bullets with about 1.5 grains of Bullseye. Sounds like a big air rifle when it goes off, and that's without a suppressor! That's really only a good load out to about 30 yards, after that the big bullet drops really fast. In colder weather, I might use a little more Bullseye to compensate for the cold air.
I call the loads "skullbusters"!
You might consider the 300 Whisper, it's tried and true for what you're thinking about doing. Good balistics, good bullet weight, and was designed for suppressed shooting. Lots of information about subsonic loads for it.