Sub for stereo receiver?

I've received different answers to this question, which I thought was basic. Admittedly, being new to audio, I know little about a lot,. But I have a Luxman R-115 stereo receiver from 1990, and would like to add a sub. The unit has two Pre Out jacks. So I asked around, contacting a local dealer, a speaker company, an audiophile friend; I also spent too much time trying to find an answer online. Maybe I just don't know where to look. What I heard:

1. One guy told me I can run cables from a sub into the speaker terminals along with my main speakers.
2. Another guy said just run cables from the sub into a Pre Out jack.
3. Another said that I should probably ditch the plan and get something that would allow me to hook up two or even four subs.
4. Another said some things I still can't understand.

I'm a simple man, and mistakenly thought this was a relatively simple idea. My bad. But I'd still like to get something to enhance my listening enjoyment, without having to buy a new power source and filling the living room with subs, and without needing to understand what bass management or DSP based EQ means. Forgive my naivety if you must; help me if you can, remembering that the simpler the explanation, the better. Thanks.