100 Hz test tone for 50 Hz hum and subwoofer localization

According to Wikipedia's Mains hum article, hum from a ground loop is usually at the second harmonic of the line frequency, because the heavy ground currents come from AC to DC power supplies that rectify the mains waveform. On a 50 Hz grid that's 100 Hz, so this is the pitch to compare your hum against. On a 60 Hz grid it's 120 Hz, so type that into the full Frequency Generator instead.

A match fits that pattern without proving it. The tone can't name the box making the noise, so that part is still up to you and your connections.

Turn the volume down first and raise it slowly, because overdriving a speaker can cause distortion. Mute the music, then listen to the buzz from your gear while the tone plays. If the pitch lines up, you've found the usual pattern. If it doesn't, try the 50 Hz tone and the 60 Hz tone, since those are the grid frequencies.

For the localization check, play the tone, close your eyes, and point at the sound. According to Wikipedia's Subwoofer article, poorly designed systems often leave everything below about 120 Hz, or even higher, to the subwoofer. The sub then plays frequencies the ear can use to tell where a sound comes from, which the article calls an undesirable localization effect. It says this is usually due to a crossover point set too high or a crossover slope that's too gentle. The article doesn't give a right crossover number, so this page doesn't either.

The article also says many small multimedia subwoofers are too small to produce deep bass in the typical 20 to 100 Hz range. One cycle of this tone takes 10 ms, and the wave is about 11 feet (3.4 meters) long.