JBL stereo issues, low volume

[DCT]

I have a 2006 Seqouia limited with the JBL system w/ DVD player. A few months ago the audio kind of quit working. At full volume, the audio is quiet. I bought a used head unit from ebay and installed. same problem. I bought a used amp from ebay. same problem. I even had the toyota dealer diagnose it, even brought in my used ebay parts for testing too. They think the amp is bad, even my new to me used one. Im trying to retain all the oem components and use my factroy dvd player. Any for sure methods of testing the amp before I buy one for $425 from the dealer? or am i better off to stop pouring money into chasing this issue and just put an aftermarket head unit in? anyone have this problem before? If i got aftermarket will I have to install an amp also?