[Wayne A. Pflughaupt]

Because I get the feeling that the more filters I create, the bigger these differences become, or are certain frequencies more sensitive to changing phase?

It’s commonly said that equalizer filters do add phase issues, obviously at the frequencies they’re affecting, but it generally isn’t anything audible.

In any event, it won’t have anything to do with the boominess problem you’re having. That might be because your sub is too flat. Typically that leads to the upper bass sounding hot, and the ultra-lows sounding weak.

Do you feel that determening phase with one speaker in rew, is accurate?. because i switched between left and right and got completely different readings. Probably because of the imbalance acousticly between my left and right soundstage though

The measurement difference is a function of the physical distance between the sub/main speaker combo and your ears. Obviously it’s going to be different measurements, because the sub is closer to one speaker than the other. There isn’t really anything you can do about it. Typically the best remedy is what brucek advised - deal with combined response.

I'm gonna change my setup and positioning next week, and put my monolith out in the open

If by “out in the open” you mean closer to the middle of the room, that seldom gets anything but bad results.

Regards,
Wayne