Don't know if this is true or not, but I remember hearing that Billy Squier (remember him!?) actually ran leads from the studio console into his car so he could hear the mixes there!
Hehe, yeah, that doesn't surprise me! Still a lot of musicians I work with like the car test, or more often these days the cheap earphone test, to check mixes I have done for them. I always indulge this, as even though I'm prefectly happy with my judgement on my monitors the client needs to hear it on a system they know well, thats normally when they say 'wow its sounding great!'. NORMALLY, that is

sometimes they pick up a real issue with the mix, often they pick up on an issue that doesn't exist or doesn't apply to the their particular track, thats when mix psychology comes into play!
The scooped sound that many like on bass doesn't really help it get heard in a mix unless the speakers have a lot of lows. You've got to REALLY get to know the mids, and work out where the bass works ABOVE the 150Hz area and somewhere below the 2k one. Thats where the prescence lives. As stated you MUST eq the bass with the full track playing. Get a notch eq and move it around untill it settles in a place just below the lows of the gtr (that should be around 250/300 give or take for genre). Adjust the gtr to line up nicely with that etc. for a clean sounding mix make sure its a distinct gap, if its a filthier track then overlap is good but it'll make the track sound a lot less distinct down there.
The low end of a mix is critical, just like piano chords the lower you go the more careful you have to be with where you place the notes, further up there is a lot of overlap naturally and you have to learn how to put certain instruments in focus without killing the others.
Aside from the all powerful eq, plugins like waves maxx bass add upper harmonics to a bass signal without drastically effecting the eq, so they sound bigger on small speakers. Worth using but only if you already have a good grasp of how to mix, otherwise you'll just flood the track with crap

Mixing is a dark art, very easy if the band plays well with good sounds and good recording, VERY hard with bad players and/or bad sounds. Good luck!