On a journey for the last couple of years

Hello @peterrock , lots of good replies!  And certainly not everyone agrees.  But two REL Carbon Specials is a great way to go (I have them so +1 @wswright20 !)

But I think and have found better amps make speakers sound better in many ways even if they can’t make them go lower. And more power is always good...more ease and effortless flow of music...With all that pretty Moon equipment I would go for the best Moon amps you feel like buying.  You can’t go wrong there, and you already have the 740P which is begging to be mated with North series amps!  It will sound better now and be ready for new speakers in the future, should you so decide.  Hello to all others with great advice!

I also don't think its yr. monos but get some Michi M8 Monoblocks, Accuphase or Luxmans (and soooo many more)  if after adding another sub and room treatment(s) you need more.  FYI, REL has very good customer support.

I would take a look at the Odyssey monobloc amps.  I have a pair of Kismet amps—in Khartago cases, and they sound superb.  About 200 watts per channel and lots of oomph. I have them driving my Fyne Audio F702s and the sound is most excellent.  The Fynes have only 8” woofers, so I have a pair of Rythmik 12” subs in the system, as well.  It took a couple of months to get them built—they can be customized, but it definitely was worth the wait.

Let me also give a thumbs up to the Soix comment about two subs.  My Fyne F-702s have only 8" woofers and the lack of bass in songs I knew it should be there was bothersome.  Soix advised me to go for two subs, and I did that, in my case going for two Rythmik F12SE subs to keep the price reasonable.  The impact on my system was amazing in a very positive sense, and it seemed to extend beyond the deep bass range.