How do you know if you need to add a sub (without auditioning one, I mean)?


I like my speakers, I like the SQ of my system, so I'm not asking this question because I'm seeking a remedy to a deficit. I just wonder if it would sound even better with a sub. and I don't want to buy/audition anything based on mild curiosity. Also, like many of us, I don't have an unlimited budget and wouldn't care to stretch it unnecessarily.
How does anyone else decide whether to add a sub or play a pat hand?
My speakers are ATC SC40v2s. By specs, they don't go low. To my ears, the bass is much more satisfying than anything else I've listened to in my limited experience.

m669326
If you're not sure about subs, why don't you try a new SB3000? That way you can return it if you don't like the improvement. If you do like it, then you can look for a second used SB3000. What do you think?
A pair of SVS sb3000s is a great choice.  I love my sb4000 and have experience with the 3000 series and I don't think you would be disappointed. You also may want to look into a pair of HSU uls-15s if the larger box size doesn't put you off. 
Please read the first response again. Notice specifically mentioned is the number of subs. Not mentioned at all is how big or powerful. That's because when it comes to subs how many matters much more than how big. So, you need four. Any size. Take your budget, buy four subs with it. That simple.
Almost any speaker can be improved with the addition of a good sub or subs (my opinion).