I think it' s arrow spine, especially if you are NOT shooting a loop (which I cannot tell from the PICS).

Some points

1) Epic 400 carbon-aeros run softer than a normal 400 class shaft, they run about .420 IIRC. The Excel' s do run true to spine listed. (EDIT they run .415 according to Easton)

2) They use a heavier front insert which weakens spine


I suggest you try a 340 class Easton shaft (even the Epic 340 which runs a bit soft too). Beman ICSH 340, ACC 3-60. Even a CX 300 or GoldTip 75/95.

My PAT DC DEFINITELY liked a stiff arrow. I had the same setup as you, 70 pounds, 28" mods, arrow length was similar, and I shot a loop, no problems with 340 class shafts, but 400 and 390 class shafts (ACC 3-49' s) were a no go.

EDIT: Because I cannot type today for some reason, and I checked the shaft spine)