Adjusting the intercom level between channels:

The MS-704 / RM-704 provides a mic gain trim at the preamp stage .. it is common to all channels.  It does not provide  individual channel mic gain trim.
 
If you have a channel where the level is lower to everyone, it is probably due to loading.  One suggestion is to go back to the termination on the channel in question at your Main station or Power supply.  If the loading is significant, shutting off the termination will increase the gain  --   If the gain is too great and you encounter feedback, you can adjust your sidetone ...  if still too great, you will want to bridge pins one and three with a 1/4 watt resistor -- start with 1000 ohms  -- If you have a resistor bridge, you can dial in various settings until you find a value that works.
 
Reminder, the Clear-Com line is looking for a 200 ohm impedance..  When you remove the termination, you are using the combination of the cable and remote stations to provide the remaining load.