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.
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.