I started using my Focal sub with my Focal Aria speakers. However after the speakers broke in I didn't need the sub.
However I found a interesting things about using a sub.
First off we all know the hassel of sub placement and the time it takes to get it right. Then to figure out what is the best crossover frequency to choose from.
Then to get the right volume for the sub and the blend with your main speakers. I was willing to do all that but the next issue I didn't think about much before.
So let's say you buy a real nice amp for your stereo. The low end on your speakers uses the amp the most. Now if you remove the bass frequencies to a sub, not your not getting your wonderful amps quality and sound coming out of your speakers anymore. the class D amp in your sub is now handling the sub power. Granted if you have bookshelf speakers and you want a sub 8 understand. But if you have larger speakers and you buy a wonderful amp/preamp, who does not want that amp to provide the sound to your music which you chose out of several amps.
In the end it all depends on the quality of the sub bass vs the quality of the amp/speaker combo. I've heard it go both ways - where the sub addition made for better bass, and where the quality of the bass in the speaker was superior.
With that being said if you buy a Macintosh, Krell, Conrad Johnson or other high quality amp that is known for certain characteristics that you want to hear and it's matched properly with your speakers then my point is more valid.
Now if your playing dance music that has a thumping bass where you want it to sound like a night club a sub would definitely be the right choice for that situation or if your amp cannot produce low bass in the room it's in. I typically don't play music over 95db due to my tinnitus