Woof Woof need a little help

'Don't overwhelm the music with the sub'....to a teen! Nice try.

You could run the sub dead end from the 'b' speaker outputs but the mains will continue to run full-range.

YOU adjust the darn thing since the youngster will doubtless crank it UP and have the crossover too high so not only will he eventually overdrive the amp.....fried tweeters coming right up, but be listening to awful, muddy ....whatever he calls it.

Get him a used powered PA sub...like a Mackie with an 18" or 15" speaker.

Many powered subs have low level and hi level inputs/outputs. These are rca and speaker connections. If the Yamaha amp does not have a pre amp out, look for a sub that has speaker connections and connect speaker wire from Yamaha amp to sub in this way. All will be fine. Some subs become more complicated by allowing a built in crossover to roll off the bass from the main speakers if you connect the mains from the sub "out"(pre or speaker). Whether he uses speaker pair B or connects it to A, the drain on the main amp is minimal to drive the subs amplifier. What main speakers does he use and what kind of room does he have ? Does your son live with you ? Because if he does, you might be introducing a situation where the bass might become bothersome throughout the house. I do not know if you like rap as much as he. There is much info on the web about subs. Hope you found this helpful.

Tape Out will give a line level to the sub, and all these subs have their own level.