[Audio 1]

I really do not think there are any cons to adding subwoofers once properly dialed in. I agree 100% with everything Ivan has stated here.

All you want to achieve is augment the lower range where the main speakers begin to roll-off. The REL's are very nice, but my only reservation with them is that they do not offer any room equalization and rely solely on physical placement to deal with any room tuning issues. That being said in most situations they do an excellent job and have a great reputation.

I recently added a pair of subs to my system and I am beyond happy with the results. I chose the B&W DB1D which has two 12" opposing drivers in a sealed cabinet.
They are controlled 100% from your phone or tablet and are very easy to set-up and control. After running the 4 band room EQ and dialing in a bit you cannot differentiate the subwoofers from the mains. I love being able to make adjustments from the listening chair.