Adding a subwoofer into my system

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. .