This needs a ton of explanation. However, i have found a very document which can help you in finding a lot of your answers.
Well, ask yourself first what generates echo. It has to do with signal feedback and often impedence mismatch.
So changing those details will affect how much echo you tend to see. The values will vary based on your signals from CO and specific equipment as well.
a 3dB variation by way is double the "volume" of the related signal.