In conventional loudspeaker system design, the force factor Bl is chosen in relation to enclosure volume, cone diameter, and moving mass to yield a flat response over a specified frequency range. For small-cabinet loudspeakers such a design is quite inefficient. This is shown by calculating the efficiency and voltage sensitivity. The frequency response is manipulated electronically in a strong nonlinear fashion, which has consequences for the sound quality, but it then turns out that systems using much lower force factors can provide greater usable efficiency, at least over a limited frequency range. For these low-force-factor loudspeakers, a practically relevant and analytically tractable optimality criterion, involving the loudspeaker parameters, will be defined. This can be especially valuable in designing very compact loudspeaker systems. An experimental example of such a design is described. This new, optimal design has a much higher power efficiency as well as a higher voltage sensitivity than current bass drivers, while the cabinet can be much smaller.