Having just added a second sub this week in part because of the rel sale, I would add to this discussion that with the good also comes a little bad. The second sub rel s/510 added richness to the sound and much deeper sound stage. The only new problem that emerged with this improved SQ is volume. While working I was listening to the streaming service I use on my $150 2.1 computer speakers and noticed that all levels were homogenized—meaning there was about the same levels of bass and treble and so forth. Playing the same playlist on my system I found myself wanting to adjust the sub level pretty often. On one song there’d be too much bass so I’d turn the sub volume down. On another song there was too little, so I’d turn up the volume, until finally I had to put the volume at some compromised position. The sub is magical and musical but I sure wish it had a remote.
Why I'm not adding a sub to my 2-way monitors for music
I've updated my blog post with more data, more analysis and the conclusion that in my particular case I do not need a subwoofer. I encourage everyone who is on the fence about adding a subwoofer or not to read and comment. I think that either way you'll be able to make more informed buying and configuration choices.
Everyone's situation is different but I hope the post helps you get to the right conclusions.