I picked up a box of 200gn factory loaded 308win subsonics by Lapua.

The report is still quite loud and I'd like to know if it could be further reduced by loading lower still.

MV, according to Lapua is 330mps(999fps)

The velocity of your load is subsonic, so you wont reduce the sound level by downloading it.
Are you using a sound suppressor?
And what type of bullet does Lapua use and how does it group?

I presume you know the extreme danger that may arise from reducing velocity by reducing powder amounts without adequate research? Rifle "blows up?" As opposed to "shoots?" If you're not familiar with the problem check out some of the authorities. Its a real live issue.