Amplifier Gain setting

I have an eclipse xa2000 that I took out of my grand am and put it in my mustang. It worked fine in the grand am but when I turn the gain up to the old setting I get a whine out of my speakers. What could be the reason for this?

Are the speakers and the head unit the same in both setups?

The head unit is the same but I put in a set of JL speakers and in the grand am I had clarion speakers

are the clarion speakers coaxials and the JL seperates using the passive crossover ?

The JL are coaxials and the Clarions were components with a crossover in the speaker wire.

the setting of gains with a DMM has fallen out of favor YEARS ago