weakest link

I'm looking to replace one piece in my system which is nak mb1s cd ,yamaha cx800u,adcom 555, and near 50m. Any opinions on one piece to upgrade?
I would definitely start with the CD player. Now day's you can purchase "superb" CD player for the fraction of the original price. I would check used Adcom 750 cd player, which would be great with your Adcom amp. Also down the road you can get the Adcom pre-amp,GFP-750. It is nothing wrong with having all one brand separates, leaves you with less chance for mistakes. Especialy, if the brand is as good as ADCOM. Good luck!
What wires are you using? I used to have adcom too and a cheap cd player.Oh boy I'll tell you the best thing you could do is upgrade you interconnects then your speaker wire. They are just as important as any componant is. I strongly recommend MIT. You'd be surprised at how much more you can get out of your system. (Try MIT terminator 2's) Its priced very fair.
I'm using aq quartz interconnect and have used aq cv4 and ocos speaker cable.