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I would check the caps on the power supply. They are rather large electrolytic capacitors that will look like they burst and/or leaking. When these die, the caps ground and the sub will make a loud buzzing sound. If you are handy with a soldering iron you can replace them yourself. Replace them with the same voltage or a little bigger and same with the size.
If the cap. says 16v 4700pF on it, for example, a 20v 5300pF or a 16v 6200mF cap. will work for this application.
Hum usually indicates a power supply problem. If it has an internal supply you will need someone to check it's producing clean DC. Alternatively if you do open it (beware risk of shock!!) initially visually check condition of /replace any bulging electrolytic capacitors.
Disconnect the input and see if it hums in the absence of an audio source. A bad audio cable shield or unwisely-routed audio cables will allow entrance of unwanted signals from external power sources, magnetic fields, even dimmer-controlled track lights. Sometimes, simply reversing the orientation of the ac power plug can eliminate humming.
If I understand this correctly: with NO input or cable attached it hums; with an RCA cable attached it works. (What does 'some' mean in this regard?)
You say that connecting the RCA cable improves its sound. That tells me you're supplying a ground through the RCA cable for stray voltage in the sub.
It seems there may be a basic power issue if it hums with no input. Sometimes a given power source in a home may be wired differently than others or may insufficient grounding, which can result in hum. Try reversing the sub's power plug or move it somewhere else.
Since the sub contains its own electronics it is also susceptible to strong external magnetic fields. Keep it and any signal cables leading to it some distance away from other electrical devices like TV's.
Sounds like the hum might be mains hum caused by the failure of the power supply capacitor(s) in the sub. It/they will be the largest on the pc board. You will find all the details (needed to replace it) on them. If you find more than one replace both.