It is not a sub being "too big" for a room that causes boom. It is the eigenmodes of the room being excited that does that, and that can be caused equally by an 8", a 12", a 15", an 18", and a 24" inch (though the lower in frequency a sub plays, the more room modes it excites. All the GR Research and Rythmik subs play low enough to excite all a rooms' eigenmodes), in a 1.0 cu.ft., 2.0 cu.ft., 3 cu.ft., 4.0 cu.ft., or 5.0 cu.ft. enclosure, sealed or ported. Employing multiple subs can help address a boomy room, as can bass traps and, feel some, DSP. The Rythmik plate amp features a single PEQ control, and adding a separate DSP unit can provide more, making reducing more resonant peaks possible.
By the way, one of the benefits of the OB/Dipole Sub is that it excites one less room mode than do monopole subs---the one of the width of the room. One less mode=less boom!