The Umik should work fine.
The problem with measuring subwoofers is you usually end up measuring considerable room effects too.
Put your microphone within about 1" of the subwoofer dust cap and that should yield a good measurement.
djn, but you do want to know how your room modes are...
measure it outside, fine, but inside is where you listen, so in the room placement is critical, you can only know that by measuring inside