My receiver says on the back that I should have 12-16 ohms impedence per speaker, if using both sets of speakers are turned on. Connected, I have one 4 ohm speaker [center, A] and a pair of 8 ohm rear [mono surroud, B]. It seems to work fine, none the less. Is there any 'danger' in doing this ? I am not even sure where I could find 16 ohm speakers.