I don't have any personal experience with the kind of load you're looking for, but I note that the Speer (#14) Reloading Manual lists some reduced loads for the 280. For example, it lists 21.0 grs of SR 4759 with the 145 grain bullet for a muzzle velocity of 1689 ft/sec out of their 24" test barrel.
A 24" barrel is a long tube to shoot subsonic loads. I shoot a 16" 300 Whisper and it takes either pistol powder, or fast rifle powder like AA1680 to obtain subsonic velocities while still cycling the action in my AR15.
With your long barrel you will have to also be careful about getting a slow moving projectile stuck in the bore. There will probably be a narrow window between just enough powder to push the bullet out of the barrel at subsonic velocities and not enough powder, leaving the bullet in the bore.
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