Hey El, to get the sound you like it's trial and error. With the 57 so close you also get "proximity" effect, basically base boost from 1 to 1 1/2 " from the sound source, good for vocals which is largely what 57s and 58s were designed for. Moving away from the source you get a more flat response (though still 5khz boost in those 2 mics, also part of their design). I agree with SC, try the bottom speaker as it has a bit more box volume. First I'd try just angling the 57 a bit toward the outside of the cone and get it off axis, while you are only pumping 5 watts it's not as big a deal (although will sound different (better I suspect)) but the 57 (while being a very versatile mic) wasn't designed for high SPL (sound pressure level), it's a vocal mic. Going off axis wont work the mic diaphragm quite as hard. I've often just hung a 57 over the top of the cab using the mic cable through a handle etc or whatever to hold it there (gaph tape...) coz I didn't have enough mic stands. So diaphragm at 90 deg to cone. Works fine and the diaphragm doesn't bottom out from the high volume.
The Senhiezer 421 was designed as a high quality very high SPL mic (~120db IIRC) and is a better mic for (loud) cabs and no 5khz boost, they are designed to be quite flat. Also very good for toms and not too bad for kick (though M88 better).
One of the best kick mic ups I did was using an EV RE20 (arguably the best "dynamic" mic ever) pointed at the center of the skin (to pick up the bottom end) and a 57 pointed at the edge of the skin (to pick up some click/tops). So what Adrian is doing with 421 and 57 is a similar concept.