from 87db to 90 db ?

An autoformer makes no difference to the sensitivity of the speaker, but it raises or lowers the input impedance, to better match to the driving amp.

Many low power amps have limited current delivery capability, so they will be able to play subjectively louder with a higher impedance load.

Nevertheless Aballs suggestion may be a good one if you can tell us the type of amp you are using, and the input impedance of your current speakers.

I'm not positive, but I think an active crossover may raise the sensitivity.

If you normally listen 3 meters away from your speakers, then shortening the distance to 2 meters will increase the SPL by 3.5 dB. It's not what you're asking for, but it does the same thing.

I didn't phrase my post correctly. Of course the sensitivity isn't changed if the modification is outside the speaker! Arthur

One thing you might want to try is a little device a friend here on Audiogon and I found a couple of years back while trying to find a sympathetic speaker for his very low power amplifier.

Of course, it will not raise the sensitivity of your speakers, but the AR TDS (forget the model #, but I could look it up if you were interested), truly doubled the volume coming out of the system. We tried it between the source and his amplifier, and it took us from being able to use 2 or 3 speakers with this amp to a whole lot more - maybe 25?

We noticed no sonic disadvantages to using this device, and we tried it in several systems. However, over time, I did find out that in some systems, it can really muck up the sound, taking away from bass and putting other things in - sort of a "robbing Peter to pay Paul" scenario. Still, you might just want to give it a shot, as in most systems, we got the loudness doubling, with no degradation. If they work for you, they definitely open a lot of doors into the type of speaker selection you outlined.