Toker, if I understand you correctly, there's an easy answer to your question: but a sub that has line outs for your current Samsons. I know that Samson makes two budget models (about $180 & $220), as does M-Audio, Tannoy, and others ($300-400). Just be sure that what you get has line outs; the M-Audio, I know, also has a variable low-freq cutoff that will send only the higher freqs to your Samsons, so there's little duplication.
I just went through this search myself and decided that the easiest and possibly best-sounding approach would be to buy a sub that's already wired up for my own small active monitors. The more I looked into external crossovers, the less I wanted to take that route. Maybe I'm just lazy. More knowledgeable folks here might know of an easy way to add a sub without line outs (like a home theater sub). For me, I decided to stay away from the home theater stuff since the prices are similar.
Best,
J.