As Alan points out.
Getting loudspeaker tone quality exactly right is really a matter of balance. Even slight variations in the relative contribution of bass, midrange and treble may dramatically alter our impressions of a loudspeaker. If you have too much bass output relative to the midrange, the mids and highs may seem a bit recessed or muted and less detailed. If a speaker is shy of bass, its midrange becomes more noticeable and we may describe its sound as “middy” or “thin” (lacking bass), because the midrange and treble are too prominent. (Alan Loft, 2010)
There is no doubt using a subwoofer "balances" the sound of a bookshelf speaker especially when that midbass is not boosted.
