Subwoofer cable question

Do as the REL recommends.

Use speaker taps with the supplied cable. Simple

+1 tablejockey.

The REL sub should have 'high level input'?

If so this is the best way to go.  Use the supplied high level cable with Speakon or Neutrik connector that mates with the high level input.  The wires go to the speaker outputs on hte amp (integrated amp) as described in the manual.  This way sounds best.

always better to connect rel subs to your amplifier high level outputs for better sound, all rel's since the early 90's have a high level connection

All subs are mono. That's just life below 80.