What do you use to CLEAN audio equipment CASES?

I use regular drug-store Rubbing Alcohol. It won't harm painted metal surfaces and will remove those nasty tobacco stains. But always test anything you use any solvent on in an inconspicuous area first to make sure. As far as black cases (like Yamaha) you can use a Sharpie (with apologies to Terrell Owens) to cover up the scratches - I did that on my Yamaha M-4 amp - works fairly well, but not perfectly.

warm water first, then alcohol, sometimes strip and spray paint over in auto shop to make it brand new.

Simple Green.
It removes oils and leaves a smooth clean finish, especially good on black components. You can get it in a one gallon concentrate or already watered down in a spray bottle at Home Depot.
The stuff cleans just about everything else in your house as well.