Which Component Needs to be Upgraded First

The amp and cd are the weaker links. That being said I would use Gunting's advice and get a good Dac and replace your player as needed.
The rest of your components can't make up for a poor source.
GBI,GBO just like a computer

IMHO, and at the risk of telling you something you already know, once you've reached your level of components, and especially since your're not considering a speaker upgrade (and I'm not suggesting that you should consider it), the only way to tell is to demo a piece--source, amp, or otherwise--in your home w/ your equipment. Beware of assuming that a newer, more expensive component will give you an upgrade in sound.

Nice speakers. I owned the B&K st-140, you could do much better.