Can I adjust the bias of my Adcom GFA 5500

The 555s were adjustable.... Why don't you ask your local dealer or Adcom? You should have the bias limit figure & a meter to do this anyway. My Parasound dealer biased my Adcom amps for me.......
Not something for an amatuer-I wouldn't try it myself. Make a mistake, you fry something in the amp-or yourself.

I know when I did a lot of modifications to a pair of 565's there was an input adjustment of some sort, it is adjustable via a tiny black square on the board that is connected to inputs on the amp. On that little black square there is a white circle with a phillips head plastic adjuster screw on it. Even with the top cover off it may be hard to see let alone adjust, I adjusted mine when the amp was going back together with the back panel just sort of hanging there. I don't recall exatly what adjustment this is but it is an input sensitivity adjustment of some sort and made a HUGE differnce in the sound when it was dialed in.