, a few tips for tracking down that null at 175hz...
You might not be able to do anything about it, in which case you shouldn't worry.
It's pretty typical.
I would want to look into it, just in case, though.
A few things to try...
Does it go away when you turn the sub off?
If so, it's a phase cancellation with the sub.
Experiment with the overall phase/time alignment, and crossover slopes (if your receiver has that option).
Hopefully it's something you can dial out.
If it doesn't go away when you turn the sub off, the cancellation is caused by an early reflection from you mains. It could be front wall, side wall, ceiling, and/or floor.
A good way to determine which surface is causing the reflection is to measure with different mic and speaker placements.
For example, if it goes away when your mic is on the floor, it may be a floor reflection.
These kinds of reflection can be difficult to fix, particularly at lower frequencies, but sometimes you can do it with speaker placement and/or acoustic panels, once you know the cause.