How to connect a subwoofer into a mono block system

Photos of your M3B show two sets of outputs.  So you could run line level (rca) cables to the sub or subs.
Alternatively, you could run speaker level from your amps to the sub.  The amp outputs look like they accept both spade and banana connectors.  So another pair of speaker cables terminated with bananas if your present cables are spades can be run from the amps to the speaker level inputs on the sub.  This method would add the amp signature to the sub.
Another option if using two subs placed relatively close to the main speakers is to jumper off the main speakers with shorter speaker cables to the subs.  The Proac terminals look like they accept both spades and bananas same as your amps so easy connection from your speakers to the subs.
Most subs only accept spades at speaker level, so any speaker cables would be terminated banana at source end, spade at sub end.

I should add the connections I suggested, would assume running the Proacs full range with the sub filling in the bottom.  This preserves the signal purity to the Proacs.
If you wanted to limit the low frequency to the Proacs, you would need to run your main speaker cables to the sub.  The another set of cables from the sub to your Proacs.

REL makes a great sub that gets its signal from the speaker leads.  It has jumper wires that plug into your speaker terminals with the wires from the amp.  These are very low level taps and won't affect your amp at all.

No love for REL subs from me.  Had one, sold it.

Thanks to you both for the guidance. What didn't you like about the REL sub, rhljazz? Which one did you have? My room is small, with very limited space for anything large, but I believe the ProAcs would benefit from bass support.

REL subs are superb, IME. Having said that, to add just one to a pair of monoblocks is not ideal. Since REL subs are balanced by design, a single sub running off the taps of the amps ( not speaker) can be problematical. REL suggest two (2) subs if a mono block system is utilized.That is how I have done it with my mono block amps.

Having two rel subs and monoblock amps I just use the Speakon sub connector with the three wires hooked up to the amps speaker terminals and to chassis ground, run the main speakers full range and just use crossover in subs to adjust for main speakers. 

acuaticdoc, I had the B3 model.  Just a single sub.  I couldn’t make it do what I wanted to hear.  I wanted to hear the “kick” in the kick drum.  I wasn’t looking for a heavy gravitas sound, with the room being so called pressured.  This type of sound seems to be what REL strives for.  Plenty of people apparently like it but I prefer a more natural presentation.
 I ended up with a pair of Gallo Acoustics cylindrical subs.  They are a sealed design and extremely compact.  They do exactly what I wanted.

I have a Rel Storm III and run it off my mono blocks with fantastic results. I had to build a harness to connect to the amps which sit on the floor next to each speaker. That harness links to the speakon connector at a junction strip. It integrates perfectly with my speakers which run full-range.

So now, after discussions with REL tech support, and the very useful contributions above, I'm left with 2 options:
1. Use separate subs, one per mono block. This would mean from a financial and size standpoint using smaller subs. I'll need to stay under $450/sub. This would acoustically bring in the amp signature, a positive.
2. Using the preamp outputs directly to a single sub. My AI M3B is a great preamp, and I'll need to prevent bass doubling through controls on the sub. But this option allows a larger sub, albeit a single.

In all a decision like many driven by economics, aesthetics and some ignorance.

If you can afford it, use two subs. I've been thru two REL subs. The Storm II and Stentor. They are really good with music. I needed two subs to even out bass response and "pressurize the room"! I ended up with a pair of JL Audio subs. If you adjust the subwoofer correctly, it fills the room properly. Bass isn't soft at a live event, it makes its presence known. Ever hear a kick drum or Timpani live?

The "pressurized room" thing actually is the "natural" state of sound and should be respected as a matter of degree of course...the low frequencies you hear as a part of the sort of "room sound" in live music anywhere seems to be the mojo missing from systems lacking a full range. "Pressurized" just sounds dangerous, but unless you get crazy with 18" driver multi thousand watt subs in your converted shipping container listening room, you should be safe.