I'd recommend before buying more gear (besides flat-frequency ear-plugs, see below), you spend at least a week's worth of time trying every possible mic position (on every speaker) of your existing cab. Believe me, moving or angling the mic a half inch can make a huge difference.
Start with the standards (as I always say, this sucks if you're the only person doing this, so try to find a buddy):
Put in flat-frequency-response earplugs, and get your head *right next* to the speakers. Move your head around until you've located approximately the best-sounding location on the best-sounding speaker. If you can monitor through your recording setup and play isolated from the cab, do so with the mic instead of your head.
Rules of thumb: if the tone is too trebly or shrill, move/angle the mic out from the center of the speaker cone. If the tone lacks presence, move /angle the mic in towards the center of the cone.