How to remove black rings left by rubber feet?

You might try mineral spirits...it's a very mild solvent. If the wood has any kind of durable finish on it then mineral spirits shouldn't hurt it (try the back side or underneath to find out). If the wood has a simple oil finish then you may remove some of the oil..then just recoat with boiled linseed oil, tung oil, Danish Oil...etc..

mr. clean magic erasers?

Have you tried citrus wood cleaner or Liquid Gold wood polish* that has always worked for me. One thing you can do to prevent future marks is remove the rubber feet and wrap them in saran wrap tightly and re-install them. It won't affect bass, mid or treble but will keep the wood clean. We all want clean wood.

* also murphys wood soap, fantastic, 409, pledge to name a few more you might have a round the house. If by chance these don't and the stain remains, the finish on the wood has been comprimised and you may have to refinish. One last ditch effort before refinishing is steel wool but that will dull it some. But if the finish is that thin you may not notice.

Soft clean rag with citrus wood cleaner should do the trick.

The orthodox method to do that is to use soft cotton or cotton cloth dabbed in cooking oil or coconut oil- very lightly as necessary. Rub gently on the spot/place. Clean off excess oil with dry, but soft, cotton cloth. Alternative to cotton cloth is the plain soft tissue (Kleenex, Puffs, etc). This method also works very well on CD covers where they put some sold/sale stickers that leave behind the glue marks. I had accidentally purchased a wrong L-bracket (Really Right Stuff) for my camera for $50. The low price was because of the adhesive on the bracket from a sticker. I eventually cleaned up the adhesive (2 mins) and sold it again for $80 + shipping :-)
BTW, oil is good for wood!!

Cooking oil works well as adhesive remover too , and not nearly as nasty as some of the chemicals I've tried . One other way to prevent this is to store some country and western CDs under your components footers . M