I had the same question and when I contacted the factory a while ago I was told it does increase the volume but it also effects the mid frequency. Being the experiental person that I am I adjusted the trim pot and to see what effect it has and yes it does effect the mid frequency range. If you change the setting too much when you turn the mid tone control it will affect the bass frequencies that is normally handled by the the bass tone control
. Be careful with adjusting this trip pot because it will change not only the volume of the instrument but it also affects the tone of the bass as well. Hopefully Ken can shed more light on this issue as I do not know all of the technical stuff about it but my ears definetly heard a significant difference when the adjustment was made.
Ken, what is the real deal on this one?