Have your bass and guitar playing at the same time, drag a highpass on the guitar slowly up until it begins to feel as if something's missing from the guitar, then pull back the highpass a few Hz. It's often much higher than you'd think.
Same for lowpass, drag it slowly down with the whole mix running until you feel there's some highend missinc, then pull it back up a bit. Also usually a lot lower than you'd think.