If the Dynaco approaches Mac then that is the way to go. Adcom is pretty good but not Mac good. In Adcom I had the 5400 and sold it after picking up the 5500. Not only did the 5500 have more power it sounded much better, the highs were smoother and more refined. I think I remember talking about this already.... any way for me the 5500 was my sweet spot in the line. I haven't heard the older 545/555 but they are supposed to be truer to Passes original design.