When it comes to EQ, I always just boost the crap out of my 800-900hz (whatever the amp supports, or just the Mids control) and if I have the control for it, I also boost my 250hz (Low-Mids). And keep in mind I've been working in 6-piece heavy metal bands with 2 guitars, 1 vox, drums, a keyboard (or two or three) and then there's me. And here's why:
<100 hz - kick drum territory (can be up to 120 hz depending on drum and drummer) no need to fight against that and muddy everything up.
27 hz - average fundamental of the Low B on a bass guitar
83 hz - average fundamental of the E on a bass guitar
100 hz - Low B on a 7-string guitar
120 hz - E string on a guitar
240 hz - Mid-range on the guitar, most metal guitarists cut this range to sound more Uber so I'm going to boost my signal here to cut through
400 hz - kick drum attack (It's the T of the Toom) normally boosted in studio recordings
800-900 hz - the Clack of my bass (sounds ugly in the room, but emphasizes your attack in the mix)
1000 hz - male vocals begin here, not my space
1200 hz - Guitar solo territory and also female vox begin - also not my space
2khz and up - cymbals and overtones, your bass doesn't go this high
Keep in mind these are general guidelines, and my reasons to approach EQ in the way that I do.
Also, the heavier the guitars get (Mesa or Marshall on 10), the cleaner you need to keep your bass to be heard, and vice versa. If you like to have a lot of gain on your front end like I do for sensitivity you may want to invest in a "cleaner" sounding amp like a GK or a Hartke - if the guitars are more laid back (Fender or Vox on a reasonable gain) then by all means - drive your Ampeg or Mesa Big Block like you stole it