If the guitars sound dense on their own, adding bass will only muddy it up more.
You definitely need to EQ your guitar tracks. Try bringing down some of the low end, around 100hz on the guitars, chances are, this will give it more clarity, depending on the kind of sound you had before recording the guitar.
Then maybe try dropping a bit around 200hz on one or two guitars, and raising 200 it a little on the others.
Then try dropping around 800hz on the lead guitars, and boosting a bit around 3khz.
These are just some suggestions, and obviously I have no Idea what your guitars sound like. These are just a few guidelines. Essentially, what you want to do is separate the guitars (your pan set-up is a good start, you might even want to try moving the -50 to -25 on both L/R channels). So you want to give the rhythm guitars a certain sound, with certain eq frequecies boosted or attenuated, to get the sound you want. But if all the guitars have the same eq, they're all fighting for the same frequencies, and will sound muddy. Try boosting a frequency on one, and lowering the same frequency on another guitar, to give them their own little space to breathe within the mix. And remember, lowering an EQ frequency is usually preferable to raising it. You may just have to raise the volume of the track a little after you're done eq'ing.
Hope that helps a little...If you want more help, check out the harmony-central.com forums. Lots of knowledgable engineers and producers post there.