My method of measuring polyphony is simply to engage a given number of stops at once (no couplers), then carefully to pile books onto the keyboard to hold the keys down until pressing any more keys causes the audio to crackle. Then count the number of keys pressed. Be careful not to release any keys accidentally because the release samples will consume polyphony and give a false reading.
E.g. 20 stops x 40 keys held down = a polyphony of 800 voices.
Martin.