extra subs will lower the distortion in the bass frequencies, and if there is more than one sub, the room modes will also be less deep (without room correction) so the bass will be more consistant.
With the 120 a sub is needed i think, with the 150 very nearfield (on a desk) it is not, from further it can help, but without sub in a not to big room, the KH150 has enough bass to give decent volume on those frequencies. The 310 is more like the 120, but can play louder and only a bit lower. A sub will help, but for a lot you can get away without.
I think more important in the case of the OP will be room correction. As far as i know the KH120A does not have dsp, while the KH120II has. So a KH120II with a pair of KH750 subs and MA1 room correction could be what the OP want. The DSP with room correction can do a lot to make the sound more neutral, also in the bass frequencies where the KH120A lacks.