Mic placement.

When recording a speaker cabinet or combo amp with a dynamic mic, apply creativity to the mic placement. 

If using a mic such as the SM57 or SM58 keep in mind it's pick up pattern. Its pattern is usually Cardioid in nature. Therefore by placing the mic in the middle of the cone you will get a not so filled sound. However, placing the mic in the lower right or left hand position of the speaker will yield better results. 

Look at the frequency response of the mic and the speaker specs, but as a rule of thumb place the mic's capsule on the lower right or left area of the speaker.