How do I know when to use a subwoofer?


Thinking of getting a Rel S/812 sub. Now have Tetra 606 $30K speakers that go down to a true 27Hz. I feel that I may need a sub and that I should be able to get more impact out of my system with one.
I mostly think of more excitement when using the system as a 2.1 theater system at night. Although it should make my music listening improved as well. I like the Rel way of allowing the main speakers to play full by the connection to the amps. However, I do like to think that my Tetras produce “natural” lows and I should be quite happy with them.
mglik
Subwoofer is fine for HT, they aren't my cup of tea for stereo listening.  27Hz should be low enough for most all music. You might like a sub and Rel makes good ones. 
Please let the OP know how many times you have heard a system with a DBA.
I’m on the same journey. So playing with options. A sub or subs can make all the difference if set up well.