Paying attention to the room acoustics is a big help. But since you don't often have control of that (or to say that if you've chosen to record in a space, you've chosen that space, so let's just deal with it!), then there are a couple of considerations for the placement of the mic:
Choose the volume. Next, find the distance from the cabinet where your body resonates with power chords and low tones (use ear plugs for best results!). Start from right in front of the amp and walk backwards to the other side of the room and listen to how the sound develops. Mark the spot.
Next, fine-tune the up, down and side-ways position of the mic. Just listen for interesting spots. I think this is best done while using headphones and sweeping the mic through the area.
One other thing to consider is to move the mic off-axis at that point. When you find that magical spot, and if you think that spot is still too bright, turn the mic off-axis a bit. Even if you feel that this brightness might be lost, consider that when recording this super-bright sound and later EQ'ing it out of the mix, you also EQ out the surrounding sound of the air. Sometimes it's better to turn the mic off-axis a bit and later add 'sparkle' via EQ later, because in essence you are adding sparkle to the entire area of air that the mic captured.
Then there's what I call the 'blast it' approach, if you are recording in a decent sounding space. If you have a ribbon mic (or maybe try any dynamic mic that can handle high spl's), try placing it somewhere within a foot or two directly in front of the speaker. Then place another mic, try a condensor, somewhere else in the room. Somewhere where you'll capture the essense of the space; where you'll record the natural reverb that's developed in the space.
Now mix those two mic's to taste.
It's all worthy of some great experimenting! Have fun with it.
Here's a link to a song that I recorded,
H and A, where all the electric parts were recorded with the first method.
And here's a link to a tune I recorded,
Trust, where I used the 'blast' method for the electric tracks.
Steve