A couple of things which I found useful to restore things closer to their original state;
This silicon spray is good for metal casings and plastic parts. It easily removes finger marks and returns faded or dirty plastic quickly to a as new state. Better than either furniture polish or window cleaner. Wipes off easily.
A non-corrosive contact cleaner, safe to use on metal and plastic. Useful to clean speaker and rca sockets, internal components such as scratchy volume pots, copper bars and other internal connections as well as fuses/holders, exposed ends of speaker cables, interconnect and power plugs with their associated fuses.
I have used these two to completely spring clean my hifi inside/out as well as restoring an AV receiver to almost new condition. They are cheaper than many 'made for audiophile' products and most probably just as good.