Unless you can generate a calibration file for the Zoom, and obtain a calibration file for its mics, any measurements you take with REW won't be of much value. You won't be able to tell where your speakers' response ends and the unit's begins.
For example, if the response of Zoom's mics roll off sharply below 35 Hz, and your sub is strong down to 20 Hz, measurements will show (no surprise) that your sub rolls off sharply below 35 Hz. Make sense?
From the manual, the response fairly flat, It works but havent got the time to compare it with ecm8000 that i just purchase, but since its can be use as a soundcard, gonna give it a go soon. It has phantom aswell, pretty handy to carry around.
The DPC Latency Checker (free) can be really handy helping you figure out what is causing the cracking & popping. The little meter will give a clear indication when drivers are taking too long to do their business, helping you figure out which is to blame.