Most common subwoofer input?

Agree with the above, however if you have a very long run to the sub or want more placement flexibility (eg/not near a 120 outlet) you might want to consider a system such as Velodyne's Subcontractor series, where the sub is separated into its two components: amp and speaker. The amp sits in your equipment rack, connected via RCA from your receiver's LFE output as mentioned above, and then connects to the separate speaker (or pair of speakers!) via standard speaker wire. Very flexible.

Thx for the suggestion!!!

Just want to make sure, I just NEED ONE RCA input (not a pair) right?
(amp -> RCA cable -> into one side of wall input -> output from wall -> RCA cable -> sub)

But I am wondering, if I don't have a receiver, and I only
have a stereo amp, can I still use RCA for my subwoofer?

Thanks!!!!!!!!!!

For an A/V receiver: yes, just the one RCA connection.
For a "stereo amp" you'd have to be more specific. For example, if it has a set of variable pre-outs you could run two RCAs, or coax cables, to the left/right imput of your sub. Or you could run a second set of speaker wire from the amp's speaker posts in parallel with the speakers, if your sub has high-level inputs. But seeing as you originally mentioned a 5.1 system, I wonder where you're going with all this. You might want to consult a custom installer, as these are fairly basic issues.

If you have a designated "sub out" on a receiver that's all you need, just 1 RCA. Using a 2 channel stereo receiver or amp you will need a left and a right coming from a line level output. I'm not sure what you are running for equipment, if it is older with no "line level out" or "main out" you may have to go with my first suggestion and use speaker cable directly to your sub's high level input, then jump over to your speakers.
Hope that helps...good luck

Before I bought a house, I rent a small room and use my stereo system to listen music ONLY. In my new house, I plan to use my system to listen music and watch movie (50/50). I heard that if I want to use sub to listen music, high-level input is better than RCA; RCA input is good for movie, is it true? should I build both RCA and speaker cable for different purpose?
Please advise.
thank you very much!!