Adding a subwoofer for me has been one of the greatest sound improvements, unless you have full range speakers a good sub will take the sound down to 20 htz. I held off myself due to concerns regarding integration & positioning. However I’ve really found that it adds a foundation to the sound, just making everything sound fuller with an expanded sound stage.
Depending on the make / model of sub you can always use a high level connection directly off the amps speaker terminals as recommended by REL or if the amp has a pre out use the low level connection using an appropriate length interconnect.
Worth thinking about what type of floor you might consider using the REL t5i with as it is down firing modal, which whilst rel claims that this boosts base relative to the size of the woofer may not be so suited to a suspended floor. Iam using SVS sb 1000 subs which have been great to my ears.