Set to a low gain, with the treble up high, put the bass where you like it and you have chimey, clean tones. Start to turn up the gain and the preamp will begin clipping just about the same time as the power amp, giving you a really nice balance of preamp and power amp distortion. This preamp has a classic cathode follower which exhibits asymetric clipping, where only one side of the wave form distorts giving an overdive with more note definition. As you turn the treble down the output actually goes up as you let more mids through.
Slightly cleaner but with more total gain to push the power amp even harder. This preamp is very simple but really lets the details through. The tone control cuts bass and adds treble as you turn it up, so for one knob it's pretty versatile.
Having the preamp on the floor allows for much easier use of the effects loop. You only need short cables to be able to run pedals on the loop. As each amp has a switchable effects loop you can run stereo effects in true stereo, or you can run it as a wet/dry rig, you can the use the volume pedal of one amp to blend the effects mix in real time.