Is this a screw-on or integral suppressor?
I ask because many integrally suppressed rifles have "vented" or "ported" barrels to bleed off excess gas, reducing velocity.
My Ruger 77-22 with an integral Ceiner suppressor has a series of holes starting about six inches past the chamber. They are in four rows (spaced 90 degrees apart), follow the rifling curve, and run for maybe eight to ten inches. There are maybe 12 holes per each row, making around 48 holes.
Theoretically, any 22LR ammo can be fired in this rifle, and be subsonic. However, the holes DO get a buildup of lead after a LOT of rounds are fired, reducing their effectiveness.
Still, after a ton of rounds through mine, they still work to some extent. I can still fire Hi-Velocity ammo through it without getting supersonic crack.
But, Standard velocity ammo is quieter than Hi-Velocity, and always has been. Perhaps due to the burn rate or amount of powder.
I have never checked with a sound meter, but to my "naked ear", I can't tell that Subsonic is any quieter than standard velocity ammo IN THAT RIFLE. In fact, the last time I compared, some Subsonic was louder than Standard velocity. Some Remington Subsonic I bought a few years ago was one of he loudest (at least it seemed) ones tried one day when testing various subsonic and Std velocity I had.
Something to remember- Sound, or "loudness" is up to the individual. What's quietest to me, might not be to you, due to the varying frequencies produced, and our hearing.
Now, after that rambling, here's what I've found in that Ruger/Ceiner rifle:
The quietest ammo has been Eley. There are several varieties and grades of Eley ammo, but most of it is very quiet in most guns to begin with. It's even quieter through a suppressor, of course. Eley had a subsonic, but it was a hollowpoint, and HP was banned in England where it's made, so it may not be avilable anymore. I don't know for sure, however. It might still be available as a solid.
CCI Green Tag has been quiet and accurate.
Winchester T22 has been accurate and quiet in that rifle. It's usually placed at least second in any accuracy tests I've done. If something is more accurate, it's by a tiny amount, and it costs considerably more. It's the same with noise. It's usually one of the quietest, but not far enough out to be worth the price difference.