How do I know when to use a subwoofer?

@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.