Small rooms have two issues: First, the reflection path lengths are short, and that’s bad because early reflections are more likely to be detrimental than later-arriving ones. Second, the smaller the room, the larger and more audible the room-induced bass region peaks and dips tend to be.
A combination of narrow and well-behaved loudspeaker radiation patterns and early-reflection-avoidant speaker setup can reduce the amount of undesirable early reflections. Room treatments can help, as long as they don’t ruin the spectral balance of the reflected energy by overly-absorbing the highs.
Small rooms tend to reinforce the bass region moreso than large rooms, so having a means of adjusting the amount of in-room bass energy can be helpful. In my opinion too much bass is more distracting than not enough bass.
In general smooth bass is "fast" bass, and the in-room bass energy can be smoothed via EQ, bass trapping, or a distributed multisub system. Which if any of these approaches make sense depends on the specifics.
Danager mentions diagonal set-up and big Altec speakers. The big Altecs have very good radiation pattern control, and a diagonal setup geometry can reduce the amount of energy in the early reflections. And if you can introduce some asymmetry such that each of the speakers is a different distance from the two nearest walls, that can help with room interaction in the bass region.
Of course the big Altecs are probably impractical for a small room at least here in the USA, but not necessarily in Japan! I mention them because they have the sort of radiation pattern that I believe can work well in a small room.
Duke
speaker manufacturer