I recently bought a Marshall 40 watt combo amp off of CL.  It's the first generation of valvestates made in '91 ish.  It plays fine...gets sound but I can't control any of the knobs but the reverb.  Thats in both channels too - from almost off to rolled all the way open nothing changes.  What could the problem be?

Well, that IS hard to imagine.

In talking about an amp, it is always good to specify the model, so I can look up a schematic.   SHould be on the front panel in small letters.   8080 maybe?

It's an 8040.  I found the schematic...I just can't understand it.

NONE works? Not even volume or distortion?
What about the contour control?
When you plug your guitar, is it loud or low volume?
What happens when you switch channels?
Are you plugging i the front panel main input or in the PowerAmpIn/EffectsReturn? (The only place where what you say would happen).
Thanks.

Are the pots broken?  Sometimes if the amp gets dropped/kicked/whatever they will break where the phenolic wafers enter the cases, then they don't do anything.

Have a good look.