Impedance of biwire-able speakers

Sean my guess is the A+B is parrallel. That means the amp is seeing a 4 ohm load. The fact that they have only an 8 ohm tap for using A and B also suggests it is parrallel. This means that the amp is not designed for anything less than a four ohm load.

I agree with Sean there is probably no sonic benefit from what you want to do.
greg

Sean hits the nail on the head. This is a question for your speaker manfacturer assuming they would tell the truth. Nominal impedance is sort of an average of impedance across the frequency spectrum. You probably can find a frequency vs impedance curve either from a review, your manfacturer or maybe your speaker manual. Once you determine the crossover point you can look at that curve and get an idea of what impedance you are facing . Actual impedance is determined based on the amps output impedance together with the speakers impedance(Thanks, Raul). Hopefully your speaker will have a relativey smove curve.
The other problem is you really need to have an separate volume control for each amp. It would be helpful to know what your amp and speakers are.
Congratulations on recognizing a point I never thought about.

Allow me to get touchy feely. To be an audiophile all you need is a desire to get the most out of recorded music. I started out with a table radio listening to FM. As I write this I am at my home office desk listening to my mini system(purchased from Best Buy under $1k).
Secondly, you are not talking about bi amping. If you are running two pairs of speakers off one set of speaker connections, you either run them in parrallel or in series. Parrallel means you connect both set of speaker wires at the same reciever speaker taps. Series means you connect one speaker to your receiver then connect the other to the first speaker. If the resulting load is to low you can cause your amp to oscillate and even blow. If the resutlant impedance is to high, you frequency reponse curve will look like a roller coaster(Raul, thus is true with tube and ss)Courtesy of another Audiogoner, here is how you calculate it:12-01-04: Elgordo
For speakers in parrallel it's R1 X R2 divided by R1 + R2, where R is the speaker impedence. So two 8 ohm speakers would be 8X8=64 divided by 8+8=16 for an answer of 4 ohms. In series it's additive so two 8 ohm speakers would be 16 ohms.
Elgordo
From your description you speakers were hooked up in paralell. Doing the math in my head that yeilds 2.4 ohms. This could indeed cause some amps to oscillate. In sereies the answer would be 10 ohms. Too high, but probably will not damage your amp. Onkyo made some of the best receivers and maybe able to handle a low load. Check your manual.

I violated a rule: The right answer is usually the simplest one.

Bi amping means TWO AMPLIFIERS. If you have a reciever with multi channel or multi zone you probably will not have a problem.
Don't worry. As you can see, I enjoy giving those explanations.
greg