Impedance of biwire-able speakers

It will vary with the individual speaker design. Given that most speakers vary impedance with frequency, it is quite possible to have two completely different loads presented to each amp. The one thing that will be consistent though is that the amp will typically see a much higher impedance at and around the resonance point(s) of the woofer and / or the vents ( if used ). It is not uncommon for a large dynamic woofer in a vented alignment to produce an impedance peak of 30 - 100 ohms. Needless to say, power transfer and control over the driver is greatly reduced in that region due to this. It is for this reason that larger woofers / designs using multiple woofers tend to benefit the most from bi-amping as the amp can now cope more efficiently with the higher levels of reactance and reduced power transfer without dragging the other frequency regions into the mess. Designs using smaller woofers with less excursion and / or designs with controlled impedance may not benefit nearly as much from bi-amping. As a general rule, most sealed and stuffed designs have a MUCH lower impedance peak at resonance, increasing both power transfer and control. Then again, sealed designs do have make slightly longer excursions than a vented design, negating some of those benefits due to slightly higher levels of reflected EMF. Sean
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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.

Greg: To take that a step further, the nominal impedance of the speaker cable is also inserted in series with the speaker as part of the load. This is why some cables are more neutral in some systems than others. Running your .08 ohm output stage from your solid state amp into a 60 - 100 ohm loudspeaker cable to feed your 4 - 8 ohm speaker seems a bit ridiculous to me, but hey, what do i know??? : ) Sean
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Thank you both, Sean and Gregadd. I am new to audigon and am finally delving into higher end audio for the first time. I have been a 'wanna-be' audiofile since my college days in the 80's, but have let marriage and kids take the front seat. I am asking about speaker impedance for two reasons. I do understand that speaker load is dynamic and subject to several factors. But, several years ago I fried one channel of my Phillips FA80 integrated amp by driving 2 pair of speakers simultaneously - (1) 4 ohm pair (KEF Q15) and (1) 6 ohm pair (MB Quart somethings...). So if I get the amp fixed and want to use both channels to drive the pair of bookshelf KEF Crest 2's I just bought, I want to make sure I dont burn it up again. Second, I have found a mint pair of KEF 104.2's and will be shopping for new components to drive them; but in the mean time don't want to mess my receiver up if I 'bi-amp' them with both channels (of my Onkyo TX8511 receiver). I am on a tight budget for a while, but refuse to do without my music. I want to experiment and learn, but can't afford to run out and buy a hi-end preamp and amps right away. I have just received my Canare 4S11 cable and want to have some fun! Thanks for your time.