I have seen subs with line level RCA inputs and believe that the purpose is for the sub to cover the Left and Right channels bass frequencies better. But what is the purpose of the speaker cable inputs on a lot of subs. I have seen some with L and R in's, and some with ins and outs. Can anyone explain why this is and what set up would need this? I am quite clueless. Thank you.

Not all subwoofers are utilized in HT setups with an AVR that has an RCA subwoofer output. Subwoofers are made that way so that people with stereo receivers or amplifiers that only have speaker-level outputs can use the subwoofer in a 2-channel system. There are also certain special instances, even when utilizing an AVR with an RCA subwoofer output, when a speaker-level connection to the subwoofer may be preferred. Subwoofers also very often have R/L RCA inputs. These, too, are for people who have stereo receivers or pre-amps that happen to be equipped with pre-amp level outputs for the R/L channels. This sort of R/L pre-amp level connection, when possible, would be preferable to a speaker-level connection in most instances.

The speaker outputs on so-equipped subwoofers allow one to connect a receiver's (or amplifier's) speaker outputs, first, to the subwoofer and then pass that amplified speaker signal on to a pair of speakers. With most subwoofers, a high-pass filter is applied to the speaker outputs. This means only frequencies above a certain value (for example, 100Hz) are allowed to pass. The net result is similar to using the SMALL speaker size setting in an AVR. The subwoofer is responsible for the low frequencies and the speakers are only burdened with having to reproduce frequencies above a certain cut-off frequency.

When a subwoofer is connected to a receiver's (or amplifier's) speaker-level outputs, none of the receiver's (or amplifier's) power is used to drive the subwoofer. The subwoofer's own amplifier still drives the subwoofer just as it would if an RCA connection were used. Likewise, when utilizing a subwoofer's speaker-level outputs, the entire amplified signal from the receiver (or amplifier) is passed through the subwoofer (usually with a high-pass filter applied) and on to the connected speakers.

awesome. now i understand. thanks for putting in an understandable way.