What are the dimensions of the room? How are you getting the dips, with the SPL meter in free space away from boundaries and far from the speakers?

Just to avoid confusion later, a wave at a fundamental frequency is also called the 1st harmonic. The next harmonic (2nd) is a doubling of the frequency of the fundamental, and is called the 1st overtone.

Usually, peaks and dips in a room are more easily heard in the bass range, with room effects dominating at frequencies less than 500 Hz or so. At the frequency of your fundamental of 1.454 kHz, the wavelength is about 15 inches, with the first half-wavelength boundaries at about 8 inches. You may need to describe you room, system setup locations and where you are measuring these dips, and how deep are the dips in dB you are talking about.

Interference (peaks and nulls) at those frequencies suggest looking at Speaker Boundary Interference Response (SBIR). Might only take some speaker repositioning or a relationship to a wall to fix.