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.