I posted my own numbers for impedance vs frequency on SLOG, I think. You can see that my new 845PXs with the AU transformer and no resistor/no capacitor never get anywhere near to being as low as 5 or 7 ohms, except above 2kHz. Having measured two different speakers, my own plus my neighbor's U2PXs, I have come to believe that there are differences among the speakers probably in relation to the many versions of the stators, the diaphragm coating, the mylar, etc, that SL has experimented with over years. (The U2PXs had been upgraded to full PX about 4-5 years ago.) Also, although I did not do any direct measurements to prove it, I believe the impedance will vary according to the setting of the bias voltage. If so, no two speakers will ever be exactly alike.
But the important point is that there is NO huge impedance peak in the curve for my speakers OR those U2PXs. (40 ohms is 5X to 8X the baseline of 5 to 10 ohms that is shown in the graph you posted.) The bass transformer, which is the same whether or not one uses the new mod, never gets anywhere near 7 ohms in my speakers (at all frequencies, the impedances I measured were higher), perhaps because I measured without the parallel resistor (which would tend to bring down the impedance as the natural impedance of the spkr is also falling with rising frequency). I did not measure with the parallel resistor in place. In fact, since a resistor would be expected to have a linear effect with frequency upon impedance, I don't see how removing a resistor could give you such a huge peak where there was none WITH the resistor, except to guess that it causes some aberrant resonance in the toroidal treble transformer, which is maybe why RW wants us to use the resistor with the toroid. This reveals an issue with the toroid, not an issue with the speaker itself.
Edits: 06/07/12
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