JBL Amplifier Problem?

I am looking for some guidance on an issue with my factory JBL system. I have searched through the forums, but am still coming up empty. Usually, I find the answers I am looking for, which means I usually do not have to post for help. Thanks in advance for any help that can be offered.

About a year ago I installed a JVC head unit in my 2010 DC. I used the PAC adapter to integrate into the JBL system. About a month ago the speakers started to cut out, it almost seemed in synch with the engine rpm's (higher rpm, more choppy the sound). Finally, the speakers just quit working. I thought it was the radio. It has been a problem for the longest time and I have been really disappointed with it. So, I ordered another radio. I was so confident, I did not even give a second thought.

The new radio arrived a couple of days ago. I immediately installed it. No sound. I checked the connections. No sound. I found my old factory radio and plugged it in to the stock connection. No sound. Everything else on the radios (display, media, etc) works fine.

I think there is something wrong with amplifier. I called around to a few dealerships today. They said there is a procedure they can perform on the amplifier, but it will cost $50 - $100. Just to check it out.

Again, I have tried searching for various topics, but cannot find a similar issue. I also cannot find any information about how the amplifier might be tested independently of any other functions.

A new JBL amplifier is expensive, but I have found a few at salvage yards. But, first, I want to be sure about my problem. I have not discounted installing a different amp/sub. I want to be able to identify the problem and weigh my options.

I welcome any suggestions or advice. Thanks again.