Check the connection at the battery ie. cleanliness, terminals tight etc. Disconnect the positive cable at the amp, whack the multimeter straight from that to BARE metal, i can't stress that enough. If there is still a low voltage test the resistance in the power cable from the amp side of the fuse. Check all connections and at a last resort the cable may need replacing. For help with diagnosing post what figures you get at each point after each test, on here.