Jonathan:
Is you amp on the floor, or raised up from the floor? As Wolf (Sonicus) noted above, high frequency waves are more directional as they are smaller and don't disperse as widely as the much larger bass waves. This is why many stereo speaker systems come with two small satellite speakers and one woofer. You can discern direction of the higher frequency waves emitted from the satellites, but not of the lower frequencies from the woofer. If your amp is on the floor, or well below your ear level, try either raising it up on a chair or table, leaning it back so it points at you (like Fenders with the silver swing-out legs on the side), or kneeling down to get your ears more on the level as the speaker. You should hear the tone more accurately.
Bill, tgo