On a journey for the last couple of years

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.

@peterrock 

Knotscott to be honest no, what would I need to accomplish that? I can google :)

Anytime you can relieve your main amps of some stress, by sharing it’s bass duty with another capable amp, there are advantages and potential for better sound.  There are always a bunch of variables, but it could take your SQ next level. There are multiple ways to approach, with pros and cons for each, but you can start simple, and expand on it.

The simplest approach would be to use a good sub or two with a high pass filter to handle the bottom end.  You can run your main amps and speakers through the sub's built in high pass filter to alleviate the strain of large power sucking low frequencies.

You could also add another amp for the main speaker woofer’s, using the current passive crossover (you’d need a method of matching volume). If you’re willing to bypass the passive woofer crossover, you could add a simple active crossover, or you could go further with DSP and a more complex active crossover.  You could also bypass the midbass high pass passive crossover on your.  However far you want to take it.