i dont understand exactly how what he did to the box reduced the gain; i was hoping someone else would have some insight as to how marc did it and chime in.
I modded my MV100 to have something like 11db-18db-29db gain rather than the stock 20-40-60 gain. I did it 6 years ago or so and of course took no pictures or notes, so I don't recall the details too specifically. You should look at the SSM2017 spec sheet (and/or the INA217 spec sheet) to get the specifics, but there is one pin on the SSM2017 that sets the gain level based on the size of the resistor (amount of resistance) on that pin. So you will need to change the gain resistors to change the gain. The spec sheet should have info detailing what level of resistor leads to what amount of gain.
The resistors are little surface mount jobbies, and can be seen in the pic above. There are resistors for one channel below the red capacitors, and I think the resistors for the other channel are up in the top portion of the pic mixed in among the capacitors (double check all this, it's been years and my memory may not be that good). You'll need to trace out the circuit (or find a schematic from Beyer) to find out exactly which resistors correlate to what.
The trick is, the design does not have 3 different resistors selected by each setting of the 20-40-60 gain switch. Instead, it is something like this: resistor A is selected as the gain resistor when the gain is set at 20. Then the gain "resistor" for the 40 setting is actually Resistor A in parallel with Resistor B. The gain "resistor" for the 60 setting is Resistor A in parallel with Resistor B in parallel with Resistor C. It makes resistor size selection a bit of a pain.
FWIW, I was able to replace the surface mount resistors with regular axial thru-hole resistors, by cutting and shaping the resistor leads appropriately and soldering them to the surface mount pads. Either way, use 1% tolerance resistors and hand match them so both channels get exactly matched resistors, otherwise the gain in one channel will be hotter than the other.
Good luck!