Electrostatic speakers

I picked up a pair of Electrostatic Research Vista by Sunrise Audio;. Hooked them up to test them and both work. One sounds significantly louder them the other with balance set evenly. Does anyone have any ideas as to why? I am running them with a pioneer receiver just to test them. They are at my office have not brought them home yet. I plan the run them with two NAD 2200 amps, biamped. 
thanks