You are probably clipping at 2 places and neither has to do with the input voltage. All movies I've seen have peaks near 0 dBfs. First, if you are doing a gain in the MiniDSP without reducing the entire level in the MiniDSP first, then you can clip the signal.
Second, any pro amp can handle 24 dBu on the inputs which is 12.28 volts. Some can even handle 28 dBu which is 19.5 volts. You aren't outputting that much voltage with your MiniDSP and UMC-1 combined. The issue is that the input sensitivity of the amp is 1.42 volts. If your gain levels are at max on the amp, then you are trying to output too much voltage. Turn down the gain knob to compensate for the input voltage.