I bought a Mesa Mini Rectifier slant 1x12 a while back and had a go at micing it with an SM57, but all I can get out of it is a very boxy sound, no matter where I place the mic.
I'd assume it is just the nature of a 1x12, but I've seen plenty of clips of people using a similar set up and they don't have the same sound. All of these people always have great recorded guitar, though, and they all seem to use two mics (one is always a 57). For example, Petrucci in any of his recent Mesa vids is using a 1x12, Marco Sfogli in his The Reaction vid is using a Mark V 1x12 combo, and Ola Englund in his V25 demo vids.
So, I don't expect to sound as good as these guys, and I appreciate they're using extra mics, but how come I'm getting such wildly qualitatively different results, no matter where I place the mic? Is it possible it's the room I'm recording in (it's very small), or is that not such an issue for close micing a cab?
Very big deal.
There's a bit of a delusion kicking round that being close to a source eliminates or reduces the room. Nonsense. You absolutely cannot escape the modal response of a room and the effect of near boundary reflections. They affect the sound at the speaker just as much as anywhere else in the room. The mic picks that up.
Move the cab around, see if you can get a more favourable interaction that way. There are lots of guides on speaker placement, as in monitors, and the physics is exactly the same for cab placement.
Also, given that your problem is currently with mids, 10cm thick of 60kg/m3 rockwool savages midrange and low midrange. It's near total absorption.