@grislybutter - agree totally
I've just added 2 Rel S812 Subs to compliment my Kef Ref 5's.
My hifi buddy Rod uses 2 subs with his Wilson Sasha Daws and when I asked him to turn them off I was quite surprised at the difference so, I experimented with my (old) Kef TDM45 sub on my system. I could see the potential but nothing like the benefits shown on Rods system using 2 subs. He kindly bought his Rel S5 subs to my house and then I realised the potential. - I had to buy some!
All my speakers are Kef so I was drawn to them first but I don't think you can connect them using 'high level' inputs directly from mono amps so that put me off. Also, after watching many John Hunter videos I thought that Rel was the way to go. they only make subs ... right?
anyway, bottom line is I have been well pleased. Bottom end bass has obviously improved but to me the most pleasing aspect is the weight now given to the mid bass and vocals and especially the guitar. Listening at lower volume levels is far more enjoyable as well, much more overall presence.
so, if you can, find a way to integrate two subs - you won't regret it
good luck