If you Bi-Amp a 4 ohm speaker = Ohms????

All depends on the crossovers, how the all thing is put togheter, but i think you should not run your amps the way you're describing. Much better would be if you run your amps as mono for lows and highs, if you connect one chanel for lows and the other for highs, don't forget there is crosstalk on each stereo unit, power supply and other factors. Question about impedance would be best resolved by contacting manufacturer of the speakers.

as far as what ohms your amps will be seeing, i'd tink ya have to contact the speaker mfr. as to the crossover construction.

as far as vertically biamping your speakers, ie: using one stereo amp per side & having the left channel drive the tweet while the right channel drives the bass/mids, many folks have done this w/good results. the possible cross-talk between channels (bass-treble) in this set-up may be less than the cross-talk between left-right channels in a conventional *horizontal* biamped set-up.

but, there's one thing i'm confused about - ewe say yure using an outboard x-over *before* the amps, then the signal gets routed thru the speakers' x-over? this i definitely wood not do. if ya wanna experiment w/active x-overs, that's fine (tho, again, i'd recommend contacting the speaker mfr as to their recommendations for x-over point/slope/etc). but if ya do this, then ewe should hook up the speaker cable directly to the drivers, & disable the speakers' built-in passive x-over.

good luck, doug

Sedond is right about the active cross-over. Only use this if the internal cross-over has been disabled (bipassed). I'm using an active cross-over, but my speakers were designed to use an active cross-over--in fact, if bi-amped they must use an active cross-over.

Using one stereo amp for left and right is not a bad idea at all. There is less cross talk in this set-up because one amp is seeing the left channel--rather than two different channels. There will be differences due to the high pass vs low pass, but they will be harmonious and I would not see cross talk as being much of an issue--if any. I presume you are doing this to get each amp very close to the speaker and run as short a speaker cable as possible--if that's the case--good choice.

Lastly, ohm rating. I'm going to go out on a limb here and guess you have a nominal impedance speaker (4 ohms), but I'm guessing you are asking this question because it has a tendancy to go very low in impedance, and you think bi-amping will help this problem. As previously posted, only the manufacturer of the speaker can give you this information. It will be different for the high end and low end. I have a pair of Martin Logan Monolith IIs. They have very low impedance from the panel. The bass has a reasonable impedance--4 ohms and probably doesn't dip much below that, but the problem with single amps on this speaker is the impedance drops from the panel, then a bass drum kicks in and there's no umpf left in the amp (umpf is a technical term). By bi-amping you isolate the demands of the woofer vs the low imedance from the panel. I don't know if you own MLs or other speakers with similar characteristics--but I said I was going on on a limb here. If this is the issue you are trying to address--it should work very well--it did for me.