from 87db to 90 db ?

Some people go with autoformers. Paul Speltz type (or something like that). They can greatly improve the sensitivity of speakers.

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