Most common subwoofer input?

Today most common inputs for sub's RCA which can connects with sub out from receivers.

RCA is probably the most common hookup. To get the sonic characteristics of your power amp you would run speaker cable to the high level input on the sub. There is no need to run speaker cable back there if you go that route, you can tap into one of your rears. Having a biwire speaker terminal and using jumpers will be the easiest way to go, then jump from a rear to the sub...or run the speaker wire to the sub first then jump over to a rear. Ahhhh, what the heck, run the rca.

the most common cord for a subwoofer is to use the RCA interconnect. That way, you can plug it into the standard subwoofer output on receivers and have control of it through the receiver.

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.