I think nomad thought you were talking about the EC-2018 microphone.
So when your echo board was installed talkback was added to that radio and that's where you need to make an adjustment. I'm willing to bet in your case it's a resistor across the pa & speaker jack which you'll need to change. The problem with that kind of talkback mod is that it's not a one size fits all type of mod. All microphones will behave differently with that mod, and in your case factoring in the echo board as well, going even further is the limiter in the radio still intact. That particular eb-2015 echo board has a gain adjustment as well which out of the box I've found to be turned up too high in my installations. You could start there, maybe it's a little hot and could create your issue as well. Play with the resistor for the talkback. I hope that helps, maybe I might have just rambled