Troubleshooting DMM not reading voltage
So I was going to set my sub amp gain with my DMM from radio shack. I unplugged the RCAs from the 4 channel, and disconnected the speaker wire from the sub amp. I turned the deck to 50/62, and tuned off all EQ, loudness, etc. Then I played my 50Hz sine wave. I set the DMM to read volts. Then I put the leads in the place where the speaker wire for the sub is supposed to come out of the amp. I got little/no reading (0.001 volts). I adjusted the gain up and down and it never changed. I put the speaker wire back in, and played the sine wave again at a low volume and the sub played it just fine. So I'm not sure why I dont get a reading on the DMM. Am I doing something wrong? I know I am looking for 31.62 volts. Any help is appreciated!
2000 Volvo v70
HU: Pioneer MVH-8200bt
Front Stage: Focal IS-165
Sub: Alpine Type-R 12"
Speaker Amp: Polk PA660 bridged to 150x2wrms
Sub Amp: Alpine MRX-M50
Sub Box: Custom, tuned to 33Hz