Hoffman's iron law states that the efficiency of a system is proportional to its cabinet volume and inversely proportional to the cube of its cutoff frequency.
If you could make a bookshelf speaker 95dB efficient at 60Hz, maintaining the same efficiency at 30Hz would require 8X the volume which you could get from doubling each of its measurement.
You'd really be better off with a floor stander that has the same footprint (it can pickup nearly 6dB from the floor) or separate sub-woofer (which can have boost from operating into a smaller space and/or a big amplifier to compensate for the small box).
If you could make a bookshelf speaker 95dB efficient at 60Hz, maintaining the same efficiency at 30Hz would require 8X the volume which you could get from doubling each of its measurement.
You'd really be better off with a floor stander that has the same footprint (it can pickup nearly 6dB from the floor) or separate sub-woofer (which can have boost from operating into a smaller space and/or a big amplifier to compensate for the small box).