How to Add a Sub on a 2 Channel System?

Thanks Eagle, the amps are Cary V12i Mono's. Using two subs, would I basically run each sub in mono, just taking another speaker cable from each amp to the single sub?....The cary's have taps for 4 and 8 ohm, I'm using the 8 ohm tap as it sounds better. Could I feed a single sub off the 4 ohm tap or is it better to keep it on the 8 ohm?
I also use a ARC LS15 that has another set of balanced outputs available and also a set of unbalanced. Would it be sonically acceptable to drive a single sub direct from the preamp?...Cost is becoming an issue, so one sub then a second in a couple months would be better provided that I don't upset the balance with only using one sub.
Thanks
Rooze

Hi,
I'm using maggie 3.6's biwired with monoblocks, what is the best way to add a sub in this configuration?
Also, is it best to leave the speakers running full range?
Do I need an active x-over with this setup, or can a sub be added keeping my existing maggie x-overs?

Thanks

Rooze

Thanks again Eagle