For a variety of reasons — some psychoacoustic/perception, some physical — low end can come across very different at different volume levels. For example, when you’re playing your guitar by yourself (sans band) at low volumes, you might reach to turn up your amp’s bass knob to give your guitar sound body and fullness.
However, when you’re playing loud, especially in a band situation, the low end of your signal can be a lot more present. At that point, juicing the bass knob won’t add body so much as potentially muddy up the low end. To keep your guitar tone’s bottom end from getting indistinct at rehearsal/gigging volumes, try turning down the bass on your amp compared to where you set it for bedroom practice. This will not only tighten up your sound and reduce “mud,” but it will also open up more room in the low frequencies for the bass guitar and kick drum in a band environment, making for a better overall sound.