You could also just buy the 336 dark series. It already has a threaded barrel and XS ghost ring sights so you can see over the suppressor. Might be cheaper in the long run after getting a gunsmith the thread it etc. As for subsonic a lot of people reload 308 using trail boss powder. without getting into the technical details it is a high volume powder designed for cowboy action shooters. Looks like most 336 have a 1:12 twist so 170 will probably be max but that is a guess. Here is Marlin link on the dark seris 336