How to hook up subwoofer

Do a search, this question comes up frequently.
If your Arcam has "pre-out," you could connect rca interconnects from there to your sub. Alternatively, if your sub has high-level inputs (speaker in), you could run a second pair of speaker wire from the speaker posts on your Arcam.
Your speakers will be receiving full frequency range signal in either case, and it will take some calibration with the crossover settings on the sub, as well as its placement to get things to sound right (lots of patience and listening).
Good Luck

You are correct. As the sub uses internal amplifier, it does not draw any real power from the amp binding posts already connected to the speakers.