ok man this took me like an hour to do at work so I hope it's helpful. and I only have MS Paint at my disposal, so the quality is uhm....a little rough.
but here's what I'm thinking. you might have some standing waves building up in the close proximity of a wall to your speaker. even with padding like arualex sometimes there's not enough distance for the sound to diffuse, so it'll reflect back and interfere with what you're trying to capture with the mic.
if it's possible to rotate your cab much how was done in my eloquent example here, that could potentially help.
or it might be a huge waste of time.
sorry
mic'ing speakers kind of sucks balls I'm realizing.
anyway here you go: