note: these cases are for bi-wired (not necessarily bi-amped). it might help.
stolen very freely from large vs small speaker settings in the FAQs
by Gene
A Rule of Thumb about Parallel and Series Connectivity of Speakers:
Parallel Connection:
If your receiver has an A / B speaker selector switch and does not mute the audio when you select both but only have one pair of speakers connected, than the connections are wired in parallel and thus when you connect two pairs of speakers in this fashion, the impedance will be:
Z1*Z2 / (Z1 + Z2)
where Z1 is speaker set # 1's impedance
and Z2 is speaker set # 2's impedance.
Note: In the parallel case, be careful not to use speakers with impedances lower than 8 ohms (nom) as your equivalent parallel impedance seen by the amplifier while both sets of speakers are running simultaneously will be 4 ohms; which may be too much for most receivers to handle.
Series Wiring:
If the sound goes off when you select A and B speaker outputs but only have one set of speakers connected, then the Receiver is wired in series. In that case, the equivalent impedance your receiver will see with two sets of speakers operating is:
Z1 + Z2
The series case may degrade overall sound quality and halve the output volume, but it is safer to drive low impedance speakers in this fashion with less capable power amplifiers. Try to use similar speakers in this case as it will help reduce phasing problems and will better cope with sensitivity issues presented by both sets of speakers running simultaneously
