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.

simple green is a good cleaner & marker's work ok to cover up the very small nick's but for the bad one's i just repaint the case,it's easy & will make the gear look like new.

if the scratches are real deep lightly sand first & wipe the case down with a damp rag,preheat your oven(yes oven)to 200 degrees & put the case in the hot oven for 10 minutes,remove the hot case & spray on as many coat's as will dry before your eye's changing direction with each coat,i get about 4 coat's on while it's still hot,let the case sit for about 2 hour's then your ready to put it back together

rustoluem makes a fantastic paint for doing this,it's called 'hammered finish' & it comes in a few different shades ranging from dark grey to coal black,the hammered finish will make it look like a pro painted it & by having the case hot when you paint it you wont get any run's or drip's plus the finish is baked on & hard as a rock & will not scratch easy or wear off with regular household cleaners.

try to figure out how to hang the case beforehand & have everything ready before you heat the case,i use a coat hanger & have it hung in advance in the garage then i put the hanger thru a screw hole,i also have a few extra can's of paint on hand incase the tip plug's on the one im spraying with.