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
Our impression of being immersed in a performance venue is dependent on more than just music. For instance, when listening to many of the performances in the Royal Concertgebouw Orchestra's live recordings, one can hear  all sorts of low frequency reverberant hall sounds and information from the stage that aren't part of the musical performance but definitely are part of the experience you have listening to live music. 

I'm a little late to your post, but late is better than never. In case you decided against adding the Rel's. I did just that. I have a pair of Tetra 606's powered by a pair of VAC Renaissance 140's. Being that the Vac's and the Tetra's are very mid-range forward in their presentation and it seemed I needed to place the Tetra's very close to the front wall in order to obtain adequate bass response. I did decide to purchase two Rel 812s subs. This allowed me to place the Tetra's 6 feet from the front wall which resulted in much better imaging. I'm 2 months in and am very pleased with the results. Best sounding speakers I've listened to so far.

Honestly, it all depends on system to system and the room and many other factors. I do agree a subwoofer may not be mandatory but is there if needed. I my situation and type of music I listen to, I feel a sub is a good addition. 

@mglik You have not mention your listening preference and size room. Also the Tetras 606 does not mention 27hz extension and at what cutoff point.