Gamo and others make pellet guns that are very effective short range, shooting at about 1600 fps. However 1600 is not subsonic. Something in the 1200 range should do the job and be fairly quiet.
Bing, 1200 fps will still have a pretty good crack to it. Pellet rifles that shoot 900 fps are quiet and more accurate than the faster ones due to the pellet not having to transition from supersonic to subsonic, which sets up a pressure wave. Good pellet rifles shooting in the 900 fps range should be capable (taking the nut behind the trigger out of the equasion) of shooting a 1" dot at 50 yds repeatedly. So, head shots are doable, and very deadly at that distance and slightly beyond, depending on the skill of the trigger squeezer.
Also with pellet rifles, a moderator (silencer) is readily available and does not fall under the ATF rules and guidelines that silencers for firearms do. So, there is no added paperwork, no $200 tax, no waiting period. I've seen some Pre-Charged Pneumatic air rifles that were far quieter than a person sneezing.