Use microphones.
A guitar speaker is designed in such a way that it rolls off the high end pretty severely. If you want to hear a crappy guitar sound, try running a guitar head through a PA or home stereo speaker. That's basically the sound you're getting when you run your guitar amps direct into the mixer without running them through a guitar speaker first.
Alternately, you can buy a speaker simulator, like the one ADA used to make, I still see those on eBay all the time... or a POD...