You are hitting on the same issue I have with the MiniDSP/UMIK-1. Even with NO filters, it reduces the sound too much. I have tried a lot of things including a Bump Box to change the voltage, opened up the MiniDSP to change the sensitivity jumpers, but regardless the output is diminished too much for me to use the MiniDSP. I did manage to get the curves flat with the MiniDSP at reference volume, but then bass is completely missing for movies…just gone. This is a subject that has been explored and is somewhat complex to tackle. Search these forums on MiniDSP gain samson s-convert.
Unfortunately, I have my MiniDSP sitting on the bench permanenly because of this.