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
Your speakers may go deep but even so you'll be surprised at the improvement if you add four subs. Not one. Forget one. Not that it won't be a bit better, but you're talking baby steps. Four is great strides. Do a search, read my review, read comments from audiokinesis, noble100, and everyone else you can find who has a distributed bass array or Swarm subwoofer system. There's no comparison. 

Its not merely deep bass. Its much more than that. Its a feeling of envelopment that comes from being able to accurately reproduce the extreme low frequency signature of large spaces and halls. Its unlike anything you've ever heard, because if you'd heard it you wouldn't even be asking about "a" sub you'd be asking about four. 

Oh and you are right about the movies. Mine is also high end audio/video and no compromises, and the effects with some movies is mind-blowing.
https://systems.audiogon.com/systems/8367
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. 
I use a single wireless Rel S/5 SHO with full range speakers and the difference is beyond satisfying.  It's very natural, seamlessly integrated, quick, tight and punchy.  Placement can be counterintuitive. It super delivers in the right location/orientation. It super disappoints in the wrong one.
Do you want your music to sound "real" or artificial?  I've heard systems that upon first impression sounded really good with tons of bass.  However, upon further listening, I determined that it was artificial.  

Real accurate reproduction, not chest thumping sound that really wasn't there when recorded.  If the speaker was designed to provide accurate bass response, then subwoofers are overkill.  Not saying that they can't be dialed in to take the load off the main speakers, but, do the manufacturers recommend using subwoofers?  That is a very good question to ask.

It's a trial and decision that one should go on very carefully.  Might be necessary depending on your room and current speakers, but most rooms and equipment where they brag about the bass response, I've found had way too much bass.

enjoy
@minorl , 
Very good points.
@OP,
The issue of the bass you are experiencing may be due to your equipment, room treatment/ or lack of, as well as cabling, not to mention speaker placement.
Rather than chase a shadow, I suggest PM'ing Johnny Rutan (audioconnection). He knows his stuff and can save you a lot of wasted time.
Yup, he is a dealer, but he won't try to sell you something, only give great free advice.
Of course, you can also post your system here, as well, but you'll get a lot of mixed information that will probably keep you as bewildered as you are now.
My 2 cents:
Trust me, trust Johnny. Of course, you can do a search here on Audiogon to confirm.
Bob
Duke, manufacturer/designer of a 4 sub array, wrote some interesting stuff a few months back about using 2 subs if you have floor standers that go deep...I agree that sub(s) will make a significant difference...

Not all music requires a sub, but or music that uses those low frequencies a sub is essential
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.