I have a very old EP350 sub and it suddenly started emitting a loud rumble with no source turned on. Tried adjusting all settings, switched power outlets (including trying it on a UPS), attached and removed the RCA cable. If I lower the volume I can still hear the rumble, but at 70% (my calibrated setting with my AV Receiver), it's unusable and I have to turn it off. Is this the end of life of the sub amp?

Try shorting the input and if it does not go away then most likely something’s up with the amp.

How do you short the input? A wire across both? And what’s the purpose, bleeding g off a charge/capacitor? Thanks

A wire between the inside and the outside of the RCA input jack. Fold the wire end going inside over and push it in no further then the inside plug length of the the cable you use. Even better if you have an old cable you don’t use where you can snip off one end strip the wires close to the plug and tie them together. Not as good but easy and not as destructive , is to plug in a short cable (shorter the better, especially if unshielded) and short the other end.

The reason for doing this is only to make sure that the input is not floating. I suspect it won’t change anything but it eliminates that question. After that if you’d need to be comfortable removing the amp to look further.

Before you perform surgery on cables or sub, have you tried plugging the sub into an electrical outlet without a ground plug? I have an older 350 as well, and it rumbles with a fully grounded outlet. I use a short 2 prong extension cord to leave the sub running ungrounded and the rumble disappears.

There is/was a ground screw on the amp plate that also can be removed to test.

Unfortunately the sub is so old that it only has a hard wired ungrounded plug (and no ground screw). Shorting also did nothing. Guessing the amp is done for.