Cedric,
I used to work for a sound company a Looong time ago. we did amp repairs and stuff as well as system installs. If you have the right test gear, it isn't too hard to do.
I would suggest you go to a site called guitarnuts2, at this address
http://guitarnuts2.proboards45.com/index.cgi
It is a forum and the guys there are pretty knowledgable on amps & stuff. They can give you some real solid advice on what to do & how to do it.
If you do the adjustment yourself, remember your messing with some pretty high voltages. be careful in there!