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

Au contraire, mon ami!

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

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.

Your comment should be clarified to say that this applies to bandwidth limited monitors. I have speakers with folded horn bass drivers. This bass is some of the best that can be had. It would be foolish to high pass LaScalas. So I do just what you advocate against, bring the subs in very low. And it works fabulously.

Not a one size fits all solution, neither is yours.

BTW, I think you meant “either” , not “neither”.

I’ve been fighting with room acoustics and a REL T9x for over a month. I was at the point of selling it until I went completely against the set up recommendations and moved it out of the corner, raised it onto a 1” granite slab and decoupled it with Herbies grungebuster pads under the feet. Now everything clicked into place, bass grip and definition, soundstage are spectacular. best of all, the relentless room resonance is gone. I don’t like the aesthetics of having treatments all over my apartment. If I ever get a dedicated listening room I’d be all for a different approach but for what I have to work with now, a sub is a phenomenal acquisition. I’m sure others are in the same situation.

I’ll just say that if you enjoy listening to monitors without a couple good subs, you either have crap subs or don’t know how to set them up.  My experience is that when you eliminate subs from a well setup system the soundstage collapses along with dynamics and it makes the sound feel like it’s severely compromised and almost broken and don’t wanna listen without the subs.  I’ve found this with full-range speakers as well, so it’s even more pronounced with bass-limited monitors.  But that’s just my experience.  There’s a reason Wilson and others use subs even with their $100,000+ speakers, so for you to say you don’t need them with some monitors — nah. 

Its all room and speaker dependent and room treatments and speaker placement.  I was going crazy with measurements.  I was able to get the Fritz Carrera Be to play -3db at 20hz and damn near flat from 23hz to 100hz in my last house.  
 

Its so room dependent and speaker placement dependent and if you aren’t taking measurements you’re pretty much shooting in the dark.