Congrats on the sub purchase! You’ve figured it out correctly: use a 1/8" stereo headphone plug to RCA cable. It’s the common “Y” splitter type of cable. Connect the 1/8" stereo plug to the “Analog Out” on the Sprout, and the RCAs to the top two jacks on your sub, labeled Line Level In. Don’t worry about the LFE-- if you were using a home theater receiver you would just connect the one RCA since LFE is a mono signal. With the Sprout we are letting the subwoofer combine the left and right channels so you want to connect both.
Our CEO Paul has a good method for finding the right spot for a subwoofer. Get a long enough “Y” cable that you can place the sub temporarily where you want to listen to music – right where you want it to sound best. Then put on some bass-heavy music and let the whole system crank. Walk around the room and listen for areas where the bass sounds nice and balanced. The best sounding spot will be the best location to place your sub.
The best sounding place is often not the best looking location, so if you’ve got to compromise on the location, you can still make adjustments with the phase control. Start it at 0 if you’re using the method above, or if you have to place it somewhere and it doesn’t sound great, flip it all the way to 180 and see if that helps.