Low output voltage from amp. Help Please!

[DylanBeatz 10+ year member]

I just hooked up my JBL p180.2. I began setting the gain with my DMM while playing a 50 hz sine wave. With the gain turned up all the way, the multimeter was only reading 4.5 volts. I was trying to match it to 41.5 volts. I then adjusted my Pioneer headunit to bring the bass setting from 0 to +6(highest it goes). That brought the voltage up to 19 volts but that is still only half way there.

Anybody have any ideas on what the problem could be? I have actually used this same amp before with my current head unit and had no problems like this. I then sold it to a friend who just sold it back to me and this is what is happening now.

I'd really appreciate some help. Thanks

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