haha i'm not a pro at this, but i've done a lot of home recordings in various ways before, so here are some tips...[you ARE recording electric/acoustic guitar right?]
your room must have a lot of reverb, thus the bassy sounds being picked up and amplified within the room. try putting the mic on a soft cushion and get direct sound from the amp. place your amp facing the mic (and mic facing the amp) and put a blanket on the wall that the amp is facing. you might also put the amp on a soft cushioned surface. don't put the mic too near the amp, the levels will be too high and u'll get unwanted distortion to the sound.