If you are going to measure stage parameter the measurement mic is usually placed 1m from the source (see "stage parameter" in help file). But if you are to measure the reverberation in the studio, you can place the loudspeaker in a corner and the measurement mic in the centre of the room. In your case I also think it will be most interesting to measure the room response from where you usually have the recording mic and place the loudspeaker where the ensemble usually are. But of course you can test different positions to measure the difference in the room response to find a optimal position of the recording mic.
Normally you don't need large signals to measure with WinMLS due to the measurement technique it uses. The Genelec have therefore more than enough power to do the measurement. Keep in mind that the Genelec is directive. If you need an accurate measurement of the room response it is recommended to use a omni polar loudspeaker.