Tony, pointing the spl meter between the speakers is about the worse way of measuring speakers. the reason for this is that you get interference between the two main direct sound waves which result in low and high spots so moving the spl meter away ad towards the speakers you will see spl variations. (particularly at lower frequencies). If you are doing single frequencies the problems get worse.
Standing waves occur in your room particularly at lower frequencies and again high and low areas particularly at lower frequencies.
If you see any speaker measurements it will be one speaker with a meter on a fixed tripod at a listening height in a semi anechoic or anechoic chamber/room.
Measuring speaker performance in any "normal" livng room is very difficult and repeating the measurements are very difficult.
the above reasons are why I am not wild on electronic room correction. These correction devices are always a compromise between getting a good sound at one point and worse sound at many other points in a room. In most cases improving sound reproducibility at one point will make it worse at another point.
For the above reasons when I made my active speakers, I used an spl meter to set the levels of the drivers at the cross over frequency and i just don't worry about the rest.
The active system I have sounds fine and not superb, because I refuse to sit at a listening point in my room.
I am qualified in room acoustics and the measurement of sound. i am not making it up. All the above are scientific facts and not my opinion.
Here is wiki's attempt at an explanation.
http://en.wikipedia.org/wiki/Loudspeaker_measurement
Michael
