Btw, REW works just fine with portable measurement rigs. In fact, I put together a portable/shippable unit for senior design last semester:
http://courses.ece.uiuc.edu/ece445/?...07&proj=18#a18 
For what it's worth, the room is absolutely going to dominate every measurement you do in the room...as such, there is absolutely some merit to measuring each speaker individually with the same amp.
However, an easier approach would probably be to put your receiver into its multichannel external input mode and then move the RCA cable to the speaker you want to measure. Most every receiver has multichannel inputs on the back, but not all. It's nice because you've already got the speaker wires run.