Sensitivity 86 v 90

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!

"would the decreased efficiency be a concern? "

Short answer: "probably not". In fact, the possibility exists that the new speakers will play louder.

Not knowing what the new (or existing) speakers are, I’m going to render a guess and predict the new speakers are larger -- i.e. bigger, and badder.

This being the case(?):

The existing speakers, by comparison, will have smaller and/or fewer drivers. All speakers have limited dynamic headroom via finite speaker piston area, travel, etc. Therefore, even though nothings sounds "wrong" with your current speakers, there is audible (and, measurable) dynamic compression which will improve when more and/or larger drivers are introduced into the equation (and, the listening room). Even IF the sensitivity of the new speakers is (somewhat) less, this could be more than made up for in gains in dynamic headroom because of more/larger drivers coupling with the air in the room.

As others have suggested, pairing your exising rig (a lot of audiophiles like to refer to their stuff as a "rig") with the target speakers may be most "efficient" way to arrive at a conclusion.

How Is Speaker Sensitivity Measured?

1. A speaker is mounted in a baffle, in a 2π anechoic environment.
2. A measurement microphone (i.e. a calibrated sound level meter) is placed at a distance of 1m away from the centre of the speaker.
3. 1 watt of power is fed to the speaker through the amplifier.
4. The measured output that the microphone records is your speaker’s sensitivity.