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

Adding subs is always an improvement if crossed over right. But each person has to decide if the cost, space and complexity is worth it to them. 

You're right. Everybody's situation is different, and the type of sub makes a difference, too. In my small 2.1 setup I use a 300w RMS 8" sealed sub that has substantially improved the sound of every bookshelf speaker I have tried.  It doesn't hurt that it has a rare high pass filter on the high level outputs.

I just added an SVS Micro 3000 to my Harbeth M30.1 based system.  Its certainly filling in space I knew I was missing.  My listening area is the right side of a 27 x 13 room and is pretty well damped.  

I can see how setting up a sub could be a PIA.  The SVS app on my android phone allows for tweaking in almost real time.  For my purposes I set the sub to "kick in" at 51hz and the volume setting at -28.  Most of the other settings I've left alone for now.  

I'd love to find an android app that can help with room measurements from my sitting position.  Any of you guys (or gal) know of an app that might be suitable?  

I guess you’re not listening to Alan Walker at concert level..;)

Musically the important thing when adding a sub to a system is that a high pass is used on the main speakers. Then as stated distortion foes down and the main speakers open up significantly. Without the high pass this won't happen and the only advantage of a sub when integrated with the room is more bass and improved sound due to the low bass supporting the entire frequency envelope. This matters but isn't as important as taking the large load off the bottom of the main speakers.