Adcom gfa 555 or 5400

Hi, new here and new to the stereo hi-fi world. I'm looking to get my first separate amp to run my two front speakers off my Denon 4400 a/v receiver. So did some research and heard good things about Adcom, and their prices seem pretty affordable for a beginner. I have a chance to buy either a 555 or a 5400 both for about $300. My speakers are Monitor Audio silver 8's. The 555 has been with the seller for ten years and he says its running great, while the 5400 is being sold by an audio technician and he claims that he's just serviced it and that its also running great. I'm not looking for booming bass....just a nice warm clear sound from my two front speakers. Can anyone point out the cons/pros between these two older Adcoms, I know that the 555 has more power, but I'm more interested in the musical aspects of either amp. Cheers and thanks!