One way to get smooth in-room bass is the distributed multi-sub system. Inevitably, each of the subs generates a unique in-room peak-and-dip pattern (and this is true for any listening location within the room). BUT the SUM of these dissimilar peak-and-dip patterns is significantly smoother than any one of them on its own.
This is one reason I have a *pair* of Golden Ear Triton Reference. Each one on its own eliminates the need for a sub, but the two of them (subs) combined sounds even better.