Try going to this http://www.aiptactical.com/Breaking_In_the_870.html and adjusting the stock as explained under recoil reducing stocks. This may not apply to yours if you don't have a recoil reducing stock. To be sure, I didn't know they made a Knox like your's looks without the recoil reducing feature but I don't keep up with tactical stuff. Now I know it makes them.
Indeed, thanks for the link! I've read much of those "points" when I got the shotgun and searched about it, but not in one place...
My recoil cheekpunching issue is more of an issue of the angle of the stock(s). With the factory stock, and the Knoxx, they both angle from the (elevated) action, to the buttpad. Rearward motion from recoil makes my face ride up a ramp, basically, not a steep one, but very quickly. Really, not a huge issue, the only time I need to "aim" is if I go to a short range weapon only hunt around here, for deer or elk, and then for only 1 or 2 shots. I can shoot it flinch free through 25 3", full power slugs, I know that, I just don't like it (and I get a mild black eye). For clays and HD, sighting just above the barrel has proven to work extremely well for me.
I think my next purchase for it will be a monte carlo type stock, if I can find one that works well with me...