Why I'm not adding a sub to my 2-way monitors for music

The frequency sweeps on your loudspeaker look good but that is not the whole story. The averaged results looking the best. They may sound good but I'm guessing not even in the same league as bigger full range speakers that could blend in a sub a little better. Not having your speakers to listen to puts me at a disadvantage but let's just say you didn't like the sub in your particular application but I would not want to paint all systems with that broad brush. 

The averaged results looking the best

@russ69

They always do, but we take them to avoid optimizing for a 1mm wide cube where our nose might be if it were in a vise. I’m not being deceptive, I’m being thorough. This is considered good practice.

They may sound good but I’m guessing not even in the same league as bigger full range speakers

Au contraire, mon ami!

Audiophiles who do this do one of two things.  They buy a sub and set it to have as low a cut off frequency as possible, which is a HUGE waste, or try to blend them in without high passing the mains.  In neither case the results are mediocre to non existent.

To blend a speaker, of any size, with a subwoofer requires the same careful planning and forethought.

I adjusted my modest Paradigm subwoofer by ear. Sounds great. Adds what I missed with my Nola Boxers. Nothing more.

Au contraire, mon ami!

Maybe I shouldn't make blanket statements without knowing more.