The difference between speaker level and line level is the level of the signal arriving at the sub.
A speaker wire connection to the sub is high-level meaning the signal has already been amplified. A powered sub is going to knock that down to line level, apply it's xover and then amplify the frequencies it is supposed to play. With a line-level connection, the signal is low level (unamplified). If the sub's xover is engaged it's just going to filter the signal to the frequencies it is supposed to play and then amplify it; if the sub's xover is not engaged, because the receiver is doing the xover, the sub is just going to amplify the signal it gets from the receiver.
So basically either approach will 'work'. The advantage of the line level connection is that bass management happens elsewhere (in the receiver). The receiver is the brains of the operation and is routing all signals to their destination. You can change the xover frequency with a press of a button on the receiver remote when the sub is connected via the low level connection. With the speaker level connection, you have to physically move the dial on the sub.
If you use the speaker level connection, the bass management settings get more complicated and you usually have to also run speaker wire from the sub back to the main speakers. That is a lot of wire and makes a mess if the front speakers are in the front of the room and the sub is at the opposite end. So, if possible, you want to use the low level connection to the sub.
You CAN use speaker wire terminated with RCA connectors and connect them normally to the receiver's sub-preout but it requires soldering the wire to the RCA connector and that is a hassle that most people would try to avoid.
