well if you're closer to the bass amp, the bass will be louder, so it's distorting your perception of the mix overall
Reading through this debate a lot of the advice seems to be about EQ and the "space" in the frequency spectrum which each instrument should sit in. That's important, but in my opinion most of the "space" in a mix is created by dynamics, reverb, and good levelling. Several people have talked about keeping the guitars and bass separate in terms of EQ, which is fine, but what if you've got two guitarists and a keyboard? There's a lot of overlap there. There's nothing wrong with two instruments producing the same frequencies, as long as it doesn't get mushy