I can tell you this, it's not the canting. My 390 Boss bindings do not cause me to wash excessively on my heel at all. And I run the full 3.5 degree cant. Do you have your straps set for flexy, responsive, or middle of the road? That could be part of your problem. Pics maybe? That might help us figure it out better.
I've tried the straps in the middle and lower positions both with the same results. Upper position I haven't tried but on another pair of bindings I tried it and hated it. Might have to try it in these. Was out today riding again and switched my bindings back to my Unions with zero issues. My Skate Banana felt normal once again which makes me believe its the bindings.
I'm convinced now more that ever bindings are subjective, what works for one person will not work the same for another. I got these on a suggestion from a friend he's riding a Rome Agent Rocker with the 390 Boss and loves them. I'm beginning to think it could be the padding, that I'm not used to riding a binding with so much base plate padding like the 390's. both the Union Force and Contact place your feet closer to the top sheet of the board than the 390's.
I'm at a loss here but it looks like I'm going to be calling DogFunk for an RA number very soon. I think I might be giving up on these and sticking with Union when it comes to bindings. Seems that's what works for me. I lihe the Canting and my knees were thankful for it too. But I'm not going to sacrific performance for comfort, that seems silly.