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maybe moving the speakers to the center of the room away from all boundaries and taking a nearfield measurement would tell me what I need to know
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Well, not completely. Certainly the size of the room has an effect. In addition REW has to be set up in such a way so that unfortunately the lower end response isn't useful.
John M says it best when he comments:
"You need to move the loudspeaker as far from room boundaries as is practical (i.e. put it in the middle of the room), make the measurement, then go to the impulse response graph and set the gate time to exclude any reflections (evident as spikes on the response, more easily seen if you select the Full Range Energy-Time curve view). The reflection-free period is generally pretty short, if you are measuring the response at 1m from the speaker the reflection free period is a little more than the time sound takes to travel to the nearest boundary and back. Usually the floor is the nearest boundary, if your mic is 1m from the floor and the speaker is more than 1m from any other boundary the reflection-free period will be about 6ms. The problem with the short gate time is that it puts a limit on the lowest frequency at which you see meaningful response information and the frequency resolution of the response, for a 6ms gate time this figure is about 167Hz (1/gate time) which means this approach is only useful for full range measurement and doesn't tell you anything about the low frequency response."
brucek