JoeArthur is right...the mid control isnt a "fixed Q" as one would expect from some amps.... if you're not familiar with parametric EQs, look here for a quick explanation.
http://www.carstereo.com/help/Articles.cfm?id=53as joearthur said, the more you increase the mids, the Q changes. not totally sure, but I'd guess that the band shifts a little up or down the frequency scale, depending on the condition of the circuit and basics like temperature and humidity.
the shelf explanation is great, joearthur
if you're having trouble with that, think if it as a high or low pass filter almost (if you're a PA geek like me, that is). Its almost like setting up two brick walls at opposite ends of the sound spectrum, where only the sounds in between are heard.
at zero, the walls are on opposite sides of the mid-range.....(drawing time!)
<LOW--------------||Low Wall||--MIDS--||High Wall||----------------HIGH>
On the other hand, at 10 (or 11 if you're really cool, because making 10 louder just wouldnt cut it) it would be like this:
<LOW||Low Wall||---------------MIDS---------------||High Wall||High>
Like joearthur said, the low and high are more like volume controls than anything else.
