I was wondering whats the difference between the Adcom GFA-545 and GFA-545II Power amps? I noticed the GFA-545II sells for about $75 more on ebay then the GFA-545.

Also whats with the GFA-555 MK2 amp? Why is this one so dang expensive compared to the 545 and 545II?

Some history on these amps would be nice. Thank you.