The only thing that is different about listening at low volume is that your hearing is less sensitive to bass, especially very deep bass. It therefore seems a no-brainer that the answer is speakers with exaggerated bass, especially deep bass. The drawback is that when these speakers are played at a more typical volume level the bass will be too powerful. This is why the best solution is for the speakers to be tonally neutral, i.e., flat, and to rely on a well-implemented loudness control to boost bass when the main gain control is set low.
I mentioned only bass, not treble, because even though our hearing isn't as sensitive at high treble as it is upper midrange and low treble, this difference in sensitivity is so slightly dependent on volume that for intents and purposes it is the same no matter the volume. Consequently, a loudness control is a bass control, but a sort of automatic bass control, and a bass control that is contoured specifically to match the way that our sensitivity to bass is different for different volume levels.
I should probably also mention that the effect accomplished through the use of the loudness control isn't altogether natural. If your speakers are flat and if no tonal compensation has been applied to the recording and if the recording mic is flat, etc., then in order for the tonal balance that you hear (with respect to bass) to match what it was for someone who was present at the venue or studio, the playback volume has to match the actual volume at the venue or studio when the recording was made. If the playback volume does not match the volume at the venue or studio, then the perceived tonal balance with respect to bass will not be the same as it was for someone at the venue or studio. The effect of the loudness control is that it allows you to have the same perceived tonal balance, with respect to bass, as for someone who was present in the venue or studio when the recording was made, but with the playback volume level much quieter or louder than it was in the venue or studio. This isn't really a natural thing. If the actual volume in the venue or studio had been as quiet as it is in your listening room when you turn the volume down, the people who were there when the recording was made would have experienced a bass-shy overall tonality, which you do not experience because you use a loudness control (or simply turn up the bass to get approximately the same effect).
Notwithstanding that the effect accomplished through the use of the loudness control isn't altogether natural, I still appreciate a well-implemented loudness control because it means that I don't have to fiddle with the bass control nearly as much, or even with the main volume control.