Sensitivity 86 v 90

@lloydc Efficiency is 1 Watt/1 meter. With this spec, the speaker is fed 1 Watt for the measurement.

With Sensitivity, the speaker is fed 2.83 Volts instead. 2.83 Volts into 8 Ohms is 1 Watt. Into 4 Ohms its 2 Watts. So subtract 3dB from the efficiency of the speaker you have now, and you'll see there's only a 1 dB difference. 

For tube amplifiers the Efficiency spec is more useful, since tube amplifier do not double power as the impedance is cut in half.

But there is another issue here, which is the how the amplifier interacts with the impedance! Your amp uses an output transformer, and output transformers are always less efficient driving a lower impedance load!

They also can lose as much as an octave of bass response between the 8 Ohm tap and the 4 Ohm tap. In addition, the amplifier will have lower distortion driving 8 Ohms.

Since the difference between the two speakers is really only 1dB (if you do the math), you won’t have any trouble in that department. So the impedance is the far larger variable. I say go for it since the amplifier will be working better (if you measure the temperature of the output transformers before and after, assuming the same warmup time, you’ll find they are running cooler; that extra heat is caused by greater signal losses in the transformer).

In short I don’t see a downside with this move.

To answer the questions, I have been using PMC Fb1i Signature speakers. 2-way, 90 db sensitivity.  Went ahead and bought PMC Twenty5.26, 3-way at 86 db sensitivity.  Based on the cogent responses here, I am optimistic this will work out ok. 
phono preamp is a Rogers PA-2.  Cartridge is Lyra Kleos, .5 mv output version. I agree with the suggestions that the phono output is probably too low.   I can increase it by changing the driver tubes from 12au7 to 12av7 or 12ax7; I will try that. 

thanks again for all the responses, this had been interesting and very informative.  I will have to read it all again several times.

12AU7 is a low-gain tube.  A 12AX7 will definitely provide more gain in a given configuration.  

Most listening takes place around 1 to 2 watts depending on speaker efficiency, distance from the speakers to the listening position, room size and a few other variables.  Musical transients can exceed that..  If your preamp / phono stage gain is low, it will give a false "reading" of the amount of amp power you're using to just get to your typical 1 to 2 watts output, but shouldn't limit transients..  

" but (as I understand it) the gain from the phono preamp can be increased by switching the driver 12AU7 tubes for 12AX7’s, if necessary."

Did the manufacturer of that preamp tell you that? If not you should find out if you can swap to a higher gain tube!

phono preamp is a Rogers PA-2.  Cartridge is Lyra Kleos, .5 mv output version. I agree with the suggestions that the phono output is probably too low.   I can increase it by changing the driver tubes from 12au7 to 12av7 or 12ax7; I will try that. 

@lloydc 

While a higher gain 12Axx tube can drop into the same socket without damage to the tube or the circuit, that's not the same as saying it will sound right. The various 12Axx tubes have different operating points; if the circuit is to bring out the best in them it will be optimized for a particular type.