Uh , I just assumed it was acoustic as I normally DI an electric through one of those LinePods or
AmpFarm in ProTools.
6" is to far too get a bass proximity effect , so try moving your mic closer in to about 2-3" and move it towards the edge of the speaker cone , and angle it outwards slightly away from the speaker. This will roll off the high frequncies and help subjectively bring up the bass.
If the amp has 2 speakers make sure that you are on the outside of 1 and well away from the other. If you are between them the two will introduce phase combing and could cancel out the bass close in if the speakers are out of phase.
A technique which can be used with 3 mics to get MONSTER wall of screaming sound guitar , is to build a tunnel on the front of your amp by putting 3 peices of covered wood together ( so they aren't reflective ) , the tunnel should gro wider as it goes out. Put a good condenser mic at the mouth of the tunnel ( about 4' from the amp ). One mic close in as I suggested above . and one mic stuffed in the back of the amp near the cone magnet pointing down towards the floor ...
SM57 is a good mic for these last two ( sorry for the confusion ).
On a mixing desk , gate the two
SM57s from the ambient mic channel so that when it gets the sound it opens up the 2 close mics. You may have to reverse the phase on some of the channels to get a good sound ... but when you get it it is sooo punchy.
All the best