Sensitivity 86 v 90

Look at the impedance dip. That is where things make a difference.

I had Salk Songtowers...easy to drive at 6 ohms and 88db efficiency with 30 watts. And great sounding speakers in their cost range. BUT it dipped into the 3 ohm range momentarily. There are subtle things that come to the fore with 93db speakers that I didn't so much with 88db. 

A lot of variables. It is correct that sensitivity on 4 ohm is normally measured with 2 volts and 2.83 volts on 8 ohms. These past couple of years, I see more and more speaker manufacturers labeling 4 ohm sensitivity @ 2.83 volts.

The amp is also a factor. Under normal conditions a good solid state amplifier can double its power output @ 4 ohms. With this scenario, you very well could hear a difference in a 4 ohm speaker vs 8.....(4db higher sensitivity plus 3db in power gain)

However, as mentioned earlier many tube amplifiers perform better at higher impedances and have separate taps for 4 and 8 ohms. In a good tube amplifier with reasonable power output and using 8 ohm taps, it is not likely that you would hear any appreciable difference. I believe that if you are not a head banger and have 40 watts per channel or more, you should be just fine with a tube amplifier and 86db 8 ohm speakers.  I hope this helps,  Tim

Agree.  I am concerned you are already sometimes listening near the top of your amp's volume range.  Running valve amps hard reduces tube life.

You don't mention which PMC speakers you are thinking of buying.  As you are using tubes it is worth looking at their impedence curve to check it is not near or below 4 ohms over too much of the frequency range.  If it is, this will put more pressure on the amp even if it has a 4 ohm tap you can use.  Some PMCs are a benign load - the fact.8 don't go much below 6 ohms.

One other thing you need to watch out for is over-optimistic sensitivity values quoted by manufacturers.  Whilst no two tests can produce identical results, in 40 years of reading John Atkinson's tests in Stereophile I have never found his sensitivity measure to be higher than the manufacturer's figure.  His measure averages 2-3dB lower.  I have seen manufacturer quotes up to 6dB higher.  It pays them to publish the highest figures they can as we all want more sensitivity.

@OP - what are the two specific models of PMC speakers you are referring to.

Rated sensitivity is only one variable and frequently, it doesn't correlate with how subjectively "efficient" the speakers appear.

Also, what size is your room and what distance do you sit from the speakers?

What phono preamp and cartridge are you using? That’s where the problem lies!