Can you describe a little more about the physical space and speaker arrangement and maybe even the rest of the system?
Since the SRM450s are a full range, active speaker, then it is unlikely that you have much speaker processing in the system, although that could be a false assumption, especially if you also have subs. You could have some polarity inversions in some cables or connectors although that would tend to affect complete sources and not just certain frequencies. Don't know what you have for a mixer or how you have it routed, so I don't know if there might be something there, maybe as simple as the related EQ on that channel turned down.
Do you have the problem everywhere in the room? Walk around the room, does the problem change as you move around, perhaps varying in level or the frequencies affected? If so, this could be a sign of interaction between speakers (I don't know how many "some" is
) or between the speakers and the room. As Bob suggested, turning off individual speakers and seeing how that affects the problem could help in assessing whether it is related to a specific speaker or a group of speakers being on at the same time. If feasible, moving the speakers a bit and seeing how that affects it might also give some information.
The room itself could be a factor. Lots of people relative to the space, a large amount of typical 1" acoustical panels, openings in walls or the ceiling, etc. could certainly affect that frequency range.
