I would think it is the subframe connection or you've bent it. But I know I have been using my FEL HEAVILY over the last few months. I can see that the subframe connection has slipped approximately 1/8" from original location by the look of the bolts and washers. If I needed it perfectly square or was through with tree pulling, I'd do this exactly. I know that the 1/8" at the frame is translating to much more 15' out at the bucket corner.
My subframe connection doesn't really have an opportunity for adjustment - it's all carriage bolts (so the bolt location is fixed). For giggles I loosened everything, moved it to level and tightened everything but it had no effect.