Adding a subwoofer into my system

Usually its the most senior/respected party first, so in this case it would be, "Mr System, this is Mr Warfdale" etc.

Or you could just connect Mr Warfdale to some wires and plug him in. Either way it is going to take some serious schmoozing for him to get along with Mr Room.

Well that's very whimsical Mr MC but I'm not sure quite how to connect Mr W's wires to Mr Cambridge or Mr McIntosh

There are a few ways to hookup the subwoofer. If you use the high level you will run another pair of speaker cables from amp to the high level inputs on the subwoofer.
If you use the low level you will run another pair of of interconnects from your preamp outputs to the low level inputs on the subwoofer!!

Mr MC, what happened to the swarm diatribe? 

HELLO SUB

Seriously what are your options?
Much time wasted around here looking uppossibilities for posters that have them in front of them.

Well Mr fuzztone don't get aggravated with me, I'm just an old school, old guy who began this stuff years ago, when it was wires from pickup to amp amp to speakers - end of.  Now it's multiple outputs and inputs and different instruction options which don't always match up with the physical reality.  I'm just looking for a bit of help here from kind people who know much more tech stuff than I do.

My personal opinion and subject to massive debate is to run a pair of front firing subs inside of your main loudspeakers. Use the loudspeaker connection if you can, it's preferred to a coax connection, seeing the same signal as your loudspeakers. Run your Harbeths full range and crossover the subs where it sounds best. If the budget requires just one sub do the same thing with one sub inside your main speakers. .