It's weird...I think they sound like azz by themselves (AXE -> K12) but not as bad thru my practice room Behringer UB2442FX mixer into the K12s. Straight sounds kinda ice picky/top end enhanced...thru the mixer somehow seems to tame that a bit.
Sure you were plugging into line level on the K12, not mic level? Assuming no added EQ from the mixer and proper gain staging I can't see how passing signal through the mixer first would change much, let alone for the better.