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.
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.