A speaker needs 2 wires - if one of these is 'earthed' hums are likely.

Such hum is oftem caused by an "earth loop" - that means that there are 2 seperate paths to "earth" in parallel and that one is of higher resistance, thus presents a higher voltage than the other - the difference of the inspired AC voltage then gets passed thru as hum.

Now "If the vol is turned up the hum isn't noticeable" seems to imply that the hum is not being amplified thru the amp that your volume control is affecting, and that it might be being picked up AFTER that amps input and volume control stage.

The hum may be caused by something "rectifying" - acting like a detector diode in a radio - a high frequency ('radio frequency', not 'audio frequency') AC signal - which can be anything - mobile phone, taxi, etc, which has imposed on it the mains hum, and thusly detecting the audio frequency hum - this can be a faulty component, or even be a 'dry joint' solder join!

To combat this, you can try leading the speaker signals thru 'choke' toroids - several windings thru such a gadget may help reduce the RF signal being fed BACK into the amp - so place these gadgets near to the amp end of the speaker cables. A dry joint (very unlikely) at the speaker may also do this too...

You mention "sub woofer" - if it is a powered amp & speaker, the "earth loop" hassle may be somewhere in there - if it just a speaker, it could be that this sub woofer is extremely sensitive to the hum frequency - there are some tricks you can do there - but that is more than I will mention now.