The PR "reacts" to the internal cabinet pressure variation caused by the active woofers.
Adding mass (washers) slows that reaction, and thus lowers the frequency.
When we used the CSS PRs in the Subs (IIRC) we started with 3-4 washers
Since the 6" woofs of the RM30 will not cause the same amount of pressure variations, I would begin with 2-3 washers and then add and listen.
Once you hear the bass you like, you can also take some "putty" and place it inside or even outside, the center cap, and pinch away for fine tuning.