Is this a phase issue? How to fix?

Are you measuring the levels from the same position in the room? I.e., at the listening position, for example?

If it is CONSISTENTLY poor through the bass it’s a polarity issue. If on the other hand, you see rippling of the frequency response at your seated position this kind of thing can happen because of different path lengths from the speakers to the microphone from different reflections.  You should notice not only dips but peaks as well.

+- 5 dB is definitely in the range of the latter issue. Polarity issues are usually wide and deep.

Place speakers close together and facing each other (a foot apart) and check phase by reversing leads on one speaker. When speakers are very close the phase issue will be obvious in a weakness in bass when out of phase.