A ground lift plug can stop hum IF it's being cause by a ground loop between two pieces of gear. Otherwise, it'll do nothing to stop hum and may even make it worse. Plus, it should be considered a "band aid fix" only. Ungrounded guitar amps are always a shock hazard, no matter how small said hazard may be. It's especially bad in old buildings where you have to use the ground lift plug to use the old two prong outlet, or in buildings that may not be wired to code. If another piece of equipment is plugged into its outlet in the opposite phase, touching the equipment or person in contact with it will give you both a very unpleasant surprise.
Thermionic