A so called treble cap will in essence work as a sort of variable highpass filter.
In other words the effect of that cap will be less pronounced the more you turn the volume up.
As to understanding the interactions of the tonestack of yours i highly recomend that you download and open up Duncans "TSC" (tonestack calculator).
http://www.duncanamps.com/tsc/Under the Marshall heading i guess you´ll learn all you could ever ask for
To adjust values just double click whatever value you want to adjust... and in turn "sweep" basically means iteration.
Much can be said for the typical Fender/Marshall/Vox tonstacks i guess....but by just fooling around with TSC you´ll get a much better understanding of what does what and why.
The real stopper of the Marshall setup that uses limited gainstages though will at one point or another be the cathode follower stage ahead of the tonestack. It by any measure lowers output impedance,which is what it´s supposed to do,however it at full volume also means that the signal becomes "hard to kill" in practice. Pushing signal much past that point will just increase upper wave compression,until the point where it all turns to mush and start sounding ugly. Many "hardcore" Marshall players therefore convert that stage to a common anode driven stage instead and thereby increase gain/distortion capacity while at the same time get a tonestack that actually DOES something with the volume turned up. Just to be considered...
Another way of limiting bass is by DECREASING the value of the cathode decoupling caps of the various stages where so applicable. ie;where for instance 22uF is installed you might benefit by replacing that with a 4,7uF instead. That stunt will have a larger effect on how "tight" the amp is perceived in the bottom end.
That decoupling electrolytes capacitance will determin at what cut-off frequency bass is limited. The lesser the capacitance the less bass.
