You should easily be able to shoot a 400 spine up to about 65#. Getting up around 70# it is going to be underspined or at the very upper edge of the charts. I question why you would drop clear down to 55# though.

My experience with carbon shafts is that they will shoot well even if grossly overspined. I've shot a 300 spine at 27" that tuned well down around 53#. You can't just drop from 72# down to 55# and expect the arrows to shoot the same. It calls for a complete retune. And with such a short axle to axle bow everything is going to be a bit more critical to any form flaws.

Most likely you're going to have to play with the centershot some. If you don't have one you would be well served to download the Easton Tuning Guide from their website. Try walkback tuning and then broadhead tuning.