The MV only applies to the sIII (TMB) channel, not both and it is a pre-PI phase inverter design. The amount of control you get from using it is quite substantial from clean to dirty. I don't know anyone who has tried a post PI MV in this amp but there is no reason it wouldnt work, but it would sound different. You might try the Lightning MV design
Theres been discussion on MV here:
http://www.18watt.com/modules.php?name= ... master+volIn the sIII, the Volume and MV control how much signal comes from one part of the preamp circuit versus the other. Sort of a Gain and Volume for that channel.
If you jump the channels, the normal and sIII channels are interactive and sound truly wonderful. There is a post here about that somewhere ...