Sensitivity 86 v 90

@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.

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.