There are changes on test server now to how AA XP is gained. It will probably be live soon.

If you have no AAs you gain AAXP 225% as fast as normal. If you already have 750, you gain AA at the normal rate. And it scales in between.

By the time you have 220, you'll be getting AA at twice the normal rate, and by 420 at one-and-a-half times the normal rate.