Sensitivity 86 v 90

"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. 

I have had various speakers and amps. Almost all amps I have had were tube of various wattages and speakers that varied from 86 db to 90db, 4 ohms, 8 ohms....you name it.

Frankly in my modest sized rooms and listening to levels in the 85-90b range, I rarely found I lacked power. I might have found that were I to have used a 20 watt amp with the 86db speakers I would have had an issue but I would never have done such a mismatch. But 100 watts per channel is lots of tube power. That being said, some speakers do work better with lots of power and then you probably would want a beefy solid state amp.

I completely agree with posters suggesting you are dealing with issues where there is not enough voltage coming from your source to drive your amp properly. You claim that you have much more output with your CD player and less with your phono preamp.

Most CD players have a healthy output of about 2 Volts and most amps input sensitivities are such that they will deliver full output with about 2V input so you have likely a good match between your amp and CD player.

The issue, I think exists within your phono preamp and cartridge. Most phono preamps have a fixed gain. They will put out a healthy voltage output if the phono cartridge has enough output. However, cartridge outputs vary a lot.

For example, If your phono stage needs a cartridge that puts out 2 Millivolts for a full output into your amp, and the cartridge you have only puts out 1 Millivolt, then that is your problem.

Perhaps you could post what cartridge and phono preamp you are using? I suspect that once you have that sorted out, you will find that the amp you have will be sufficient to drive those 86db speakers rather well. I think a change in your cartridge to one of higher output or a change in your phono preamp to one with more gain (some phono preamps have variable gain switches and are adjustable in fact) will probably improve the phono side of things irrespective of what you do at the speaker end of things

Many in this hobby, piece their systems together, buying from various sources online. What that approach lacks, if you don’t have the experience, is the knowledge and expertise gained from a brick and mortar dealer who would guide you through the process to ensure component work well together. I am not a dealer, but knowledge and experience goes a long way towards helping avoid these mistakes. Forums are useful, but as you know, everyone has an opinion, and many times they are wrong.

@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.