I haven't been following this thread, but interesting stuff to stumble on... I added a 57 a few months ago and am finding the same thing, when paired with my 906, which I use for almost everything. The 906 almost sounds a little scooped to me, it has a decent amount of bass and high sizzle, but sometimes "misses" the midrange I hear in the room. On the other hand, the 57 is a little honky on its own, almost like a slight cocked wah vibe. So I'm experimenting with positions and levels using them together. So far, they sound best with the predominant tone being the 906, and the 57 attenuated about 6-7dB, and the 57 fills in what the 906 is missing.
The last couple songs our band recorded were done with a pair of 57's, and while it sounds good, it's just a little honky. I'm going to push for more than just 57's next time I hit the studio.