Taking the cap off the 58 was a good suggestion. Also trying to back off the distortion a bit. I find that the crispy high end distortion records a bit too harsh most of the time. To get around this sometimes I'll just back off the drive or dist. gain a bit and the recorded track comes out much cleaner.
Also, don't be afraid to get the mic as close as you need, just back off the preamp (mixer trim) to adjust recorded levels. Often guitars sound thin when the mic isn't in the right spot. Too far=less bass more room sound. Too close, well you almost can't get too close. I've miced a relatively quiet amp 1/2" off the cone wall. The important thing is to have someone move the mic's axis and proximity slowly while you monitor from elsewhere. At the very least move the mic carefully while listening to the mixer through loud 'phones. Of course as stated above always check the recorded sound, thats what matters.