Does impedance of a speaker change when one bi-amps?

Being able to get rid of the speaker-level crossover parts is one of the two reasons for bi-amping (doing so has at least the potential for producing a worthwhile improvement in the sound quality of a loudspeaker). The other is, as Roger Modjeski said above, to separate the low frequencies from the mids/highs in the amplification (giving each it’s own amp, of course), thereby decreasing the potential for the creation of IM distortion in the amp(s).

Bi-amping need not be complicated and difficult. And, it can be cheaper than replacing stock speaker-level x/o parts with those pricey boutique ones. Not only that, you can use a brute-force amp on the woofers if you want, with a nice tube amp for the mids and highs. I first bi-amped with a pair of Magneplanar Tympani T-I loudspeakers in 1973, using the ARC passive x/o designed specifically for that task. Nelson Pass makes a great electronic x/o, but it’s not cheap. Fortunately he also makes (or made, it has just been discontinued) a modestly-priced 2-way x/o, the First Watt B4. It provides 1st/2nd/3rd/and 4th order filters in 25Hz increments from 25Hz to 3200Hz. All discrete (no opamps, no ic’s), retail price $1500. Reno Hi-Fi may have one left, I don’t know. For DIY’ers, it will soon be available as a kit

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  For a long time I've had trouble matching up Stereophile reviews with my experience of the same speakers. I think I've found the reason why. They aren't reviewing speakers at all. They are reviewing hearing aids pretending to be speakers. This is why they are so expensive. What I mean is that the speakers Stereophile praises would only sound good to some one with hearing loss between 7kHz and 15 kHz, which I lack. It's clear that completely different manufacturers have taken advantage of this "trick."

I'm calling this trick the "Stereophile Curve" and the more I go back in time to look at megabucks speakers rated highly, the more apparent the truth of this curve becomes. Of course, the alternate, benign explanation is that the reviewers have all bought the B&W studio heritage hype, and they have become accustomed to thinking of the B&W 800 series speakers as a neutral reference, which, objectively, they can't be.

IMO, parametric equalizer pair with active crossover can make a huge improvement!

The most interesting loudspeaker product I am aware of is Roger’s new Music Reference ESL loudspeaker/dedicated direct-drive OTL amplifier combination (a great idea, though audiophiles don’t like to have their amp dictated to them ;-). ESL transparency, speed, lack of coloration (unsurpassed vocal and instrumental timbre, texture, etc.), and all the other ESL attributes (okay, I love ESL’s!), and no input (step up) transformer. And driven by a dedicated, direct-drive OTL amp (no output transformer, the tubes connected right to the ESL panels). What could be more pure?! A powered sub is included, and since not a single other power amp or other electronic component is required or necessary, you just feed the system the signal of your choice. The combined price is less than that of the multi-driver/complex-crossover monstrosities reviewed in Stereophile every month. I’m saving up for my pair ;-) .

There are companies making powered monitor loudspeakers for the pro sector (Westlake Audio, for one), but if you have heard any of them you know they aren’t exactly audiophile products, made for listening to music in the home. Sensitive and dynamic yes, but too colored, shouty, and coarse for me.