To facilitate seeing through the grill cloth, use a flashlight.
This position produces the most high frequencies and moving the mic closer increase both level, low frequencies and reduces the cabinet’s contribution to the overall sound.
Some guitarists and engineers say there is nothing coming out of the exact center of a speaker that’s worth recording. (I could make a funny retort here too easily.)
This is true for certain speakers, but test this position yourself to judge its usefulness.
If you want less highs and more warmth, move the mic sideways, parallel to the floor, toward the outside of the speaker.
Move in 1-inch increments, with someone in the control room you trust listening for sound changes. With a mic positioned inches from a speaker cone, small increments make a big difference.
Figure 3, click to enlarge
At An Angle
Figure 3 shows an SM57 at a slight angle on a Fender Brown-Face Deluxe. I find this a better position most of the time since you seem to get plenty of highs and more tonality than straight on.
It’s a good idea to try from both sides and from the top or bottom. If the mic ends up on the floor pointed at the speaker, and sounds good, nail it down! The floor is going to trap bass frequencies and also add tone especially if it is made of wood and is built on a raised foundation.
Tilt The Amp
If the floor seems to “close down the sound,” try tilting the amp back. Fenders have chrome legs on the sides of the cabinets for that purpose, and it also projects the sound up at the musician. VOX amplifiers, like the AC30 and Super Beatles models, come with tilting carriage stands that completely isolated the amplifier from the floor.
If possible, I like to set the amp on a folding chair.
Figure 4, click to enlarge
Figure 4 shows an MD 421U on a vintage Fender Tweed Deluxe. Here, the amp is tilted back so that the bottom of the amp’s cabinet couples less with the floor.
All of these amps are open-backed, so a wall or open space directly behind them greatly affects bass response.
Front & Back
Figure 5 shows an open-backed amp, the Matchless DC30, with an SM57 on the rear and a KSM44 on the front.
Figure 5, click to enlarge
This set-up produces a very unusual tone and allows mixing the two mics for a mono track or using two tracks placed left and right in the mix. The phase of one of the mics should be flipped.
Try moving the mics very close to the speakers and processing the rear mic through a very short delay - less than 3 ms (milliseconds). Also, try NOT flipping phase.