Could the checkboxes be replaced with radio buttons?
If you choose G, then M/A/A+ are off.
If you choose M, then A/A+ are off.
And so on.
It just saves on a bunch of clicks if you're trying to go from viewing A/A+ things to viewing say G/M items. I am sure I am not the only user who is accessing primfeed in locations where you have privacy at times and much less privacy at other times.