Thanks, brucek
You have a way of cutting through the swirling mists of doubt to bring clarity where none formerly existed.
I once did a soundcard calibration but it was so flat I decided it wasn't worth worrying about. (Soundblaster Live! and a 1 foot long cable)
As I use a 10 meter cable between the SPL meter and the soundcard and another out to my preamp I had better include those in the soundcard calibration. 60 feet of cable might have some visible effect in REW. We shall see.