It's not uncommon for different channels on an amplifier to have slightly different volume levels, especially if they are designed to provide different tonal characteristics. However, if the volume difference is significant, it could be due to differences in gain staging or circuit design between the two channels.
super mario brosThe gain controls on each channel (Volume and Gain) interact differently with the preamp and power amp stages of the amplifier. Experiment with adjusting the settings on both channels to achieve a more balanced volume level between them. You may find that tweaking the Gain control on the sIII channel, in particular, helps to better match its volume to the normal channel.
Since you mentioned using a Post-Phase Inverter Master Volume (PPIMV), it's possible that the PPIMV settings are affecting the overall volume balance between the channels. Try adjusting the PPIMV control to see if you can achieve a more consistent volume level when switching between channels.