I have chrome finished amps so I can use Brasso (put onto cloth & never directly onto amp). Some elbow grease & the thing shines!
Previously for matt finished amps, I used a damp cloth for simple finger prints, 71% alcohol for greasier marks. Then to finish off, a bit of Pledge on the face plate & 1-2" on the top panel & side panels! Yields a high-gloss finish. Of course, use Pledge sparingly & again put onto cloth & never on amp directly. Avoid the heat fins, vent slots & the rear panel.