Is the band a folk band or a rock band? ie does it have lots of equipment, or just a little? Because if there's lots of equipment, any item of it could be causing the hum

Do we really know that the hum is coming from the amp itself? Has any checking been done?

Speaking as a practical female, I would do the following:

1. Read this, then follow the steps with the grandson

http://www.ehow.com/about_6600364_guitar-amp-buzzing_.html

There are other articles on the Net with cheap, practical suggestions, which you can find by Googling 'what makes amplifier buzz.' You could find out that the problem is merely a hinky electical outlet.

2. Other practical tips from my husband - don't let cables cross one another. Don't pile excess cable in loops. Don't let speakers get too close to a microphone. Don't let a microphone get in the path of a speaker.

3. If this self-help doesn't do it, I'd e-mail the people who made the amp and ask for a recommendation on where to take it.

FYI, I looked at the article linked in the second post. It was Greek to me, and on the bottom of the page it was dated 1998.