From what I have seen I would put it in the 5 series. As far as what Charlest said, I'm not sure I'd label it angular, because from the Cyclones I've tried they tend to start up a fair amount sooner than the 508A. However, I would put the total hook and backend reaction the ball produces in-between the 508A and 503C.
Overall, it's a very good piece. Especially considering it's price.