This has been bugging me for a while now. I've noticed that my guitar sound in the front of my amp is very different from the sound I hear when standing over the amp (I mean when the amp is on the ground, not slanted or anything). When I'm standing, the sound is really tight and nice, but when I sit down or face the speaker straight, the sound gets either really brittle or mushy and muddy. I'm not that good at tweaking sound from my amp, so could anyone explain why this happens and tell me how to get my guitar/amp to sound as if i was standing over it.

Any kind of help appreciated!
"Everybody must get stoned!" - Bob Dylan

The positioning you are in relative to the speaker can have a huge effect on the sound of the amp. Treble frequencies are more directional than bass frequencies, which means that as you change your positioning in the room, the way treble frequencies are perceived changes too, hence the effect you're experiencing. The room itself has a huge effect as well, as frequencies can be absorbed and/or reflected off walls and furniture. And again, it's the higher frequencies than tend to get filtered out.

There's no way to actually fix the problem, but rather to try to compensate for it by adjusting your EQ for the positioning you're in relative to the speaker as best you can. Some people are particular to the kind of grill cloth used on cabinets because of this effect. A lot of people like thicker cloths as it helps remove a lot of the piercing high end frequencies.
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Last edited by T00DEEPBLUE at Jul 8, 2013,
It wouldn't be that big of a problem by itself, but the problem gets disturbing when my amp gets mic'd up at gigs. The guitar sounds really brittle and harsh and is rather horrible through the PA.
"Everybody must get stoned!" - Bob Dylan

That happens because less high-end frequencies are being filtered out by the room you're in as you're micing the speaker almost directly.

Getting your mic in the right place is an art. The best place depends on what you like tonally, but generally the further you are away from the cone of the speaker, the less high end. Moving the mic a couple of inches away from the cabinet can improve the bass, but reduce your attack.
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All right, thanks!
I suppose I should sit down with my laptop/sound card and just record the sound and try different placements.
"Everybody must get stoned!" - Bob Dylan

That would give you a good ballpark.
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You didn't mention what kind of amp you're running. In particular, closed back 4x12s beam treble beginning as low as 500Hz. A single 12" speaker in an open back combo, not so much.

The "muddy" business can happen because of acoustic or mechanical coupling.

If your amp is sitting on a hollow stage, the stage itself can resonate and act as a chamber that will produce muddy bottom end. That's mechanical coupling. You can reduce or eliminate that by putting the amp on a piece of foam (like the Aurelux Gramma acoustic foam pad) or a big thick packing blanket, isolating it from the stage.

Acoustic coupling is another matter. Bass frequencies can be overemphasized and sometimes muddied due to reflection from the floor or a wall next to your amp. Here's a better explanation of both acoustic and mechanical coupling: http://barefacedbass.com/technical-information/stage-or-floor-coupling.htm

I put my combo amps up on an OnStage RS-7000 amp stand (sturdy, holds 150 lbs, adjustable for height, etc.). This reduces both mechanical and acoustic coupling and allows me to hear what's coming out of the amp, rather than what's coming out of the *side* of the speaker.