Originally posted by bobbypunk
if you need to do it it may work from R+ and L- or R- and L+
but it won't sound great. Tell the amp there is no sub.
I wouldn't recommend you connect your passive sub to a left and right speaker terminal. If it doesn't actually cause damage you will not be getting a proper full range signal to drive your sub. The sub will be seeing the difference between the channels. Rather like the old Hafler surround sound system.
A passive sub has to be driven by an amplifier via a speaker cable from one set of speaker terminals. If you don't intend to use a centre speaker you might be able to use that channel to drive your sub. But if you are getting a centre speaker forget the passive sub and start saving hard for an active sub.
I really can't recommend that you try to drive your passive sub in parallel with one of your main speakers either. It is one hell of a load on that amp channel. Not only would the amp be seeing a very low impedance but you would be trying to move a lot of air with two speakers using the full frequncy range. That takes an awful lot of power from the amp and it won't like it.
Passive subs work best when running in parallel with carefully matched small satellites or driven from their own dedicated channel of a power amp. Or a completely seperate power amp. But you'd need 2 pre-out sockets from your pre-amp to do that (I believe).
NIMBY