The input impedance is given as 40K for a balanced connection.

Just use the balanced Y connectors to split the signal to each pair of amplifiers.

That will be a 20K ohm load on the driving circuit, which is not a problem.

That Kramer unit will probably degrade the sound quality IMO.

Connect the subs directly to the LF terminals of the speakers with #18 or #16 speaker wire and use the speaker-level sub input terminals (do not connect the balanced inputs). The current to the sub input is quite small, so the wire does not need to be any larger than that.

That will ensure that the exact same signal that is going to the LF part of the speakers is also going to the subwoofer.

That is the best way to connect a subwoofer.

I'm surprised you need subs at all?

Especially with excellent amplification.