Would like to add a sub to a two channel system

I'm hoping someone can give me advice on what would be a cost-effective way to add a subwoofer to my stereo setup which currently employs two bookshelf speakers. I'm running a B&K ST140 for power, an Adcom GFP preamp, a Music Hall MMf-5 TT, and Paradigm Atoms for speakers. I have a small sub I'd like to add. It is amplified but can also be powered by the main stereo amp. I would like to get advice on cable options or (less desirable) to buy some kind of crossover or ???
I humbly await for assistance.
It would be great to get more info on the sub to be able to give better advise.. brand and make?
Most likely, based on your info, you can either use regular interconnects that you hook up to pre-out on the Adcom (It does have more than one, I believe), or use speaker cables from the B&K and hook up to the "speaker in" or "high level" inputs inputs if your sub has them.
Search for "subwoofer" and look for "SMS" posts in the speaker threads. I have commented pretty extensively on the outstanding Velodyne SMS-1 sub controller. I am a true believer.

My opinion is to skip the external crossovers, etc., for now and just use the sub's built in crossover set around 50 Hz, but do try to find a second sub identical to the one you have. I have found it much easier to integrate two subs rather than one for various reasons.
Thank you Arni, Marykl and Dan_ed! I downloaded a users manual for the Adcom preamp and learned that indeed both sets of pre-outs can be used simultaneously. So I will connect the "extra" set to the "low-level" inputs in the sub,and set its crossover to 50-60Hz and see how this works. I will also look up the SMS sub controller to learn about it. I'll report back as I try my options. Again, thank you all for your help!