An EQ (either in the loop if you're just running out of an amp, or on the board if you're mic'd) is a big help.
You want low mids. You'll get volume and separation without sounding nasally or tin-y.
Angle of projection doesn't hurt either. If you're using a 2x12, elevate and angle it. If you're using a straight 4x12, tilt it or swap it for an angled 4x12. If you're already using an angled 4x12, raise it up.
If you're already doing all these, tell your drummer to calm the fuck down.