What should I upgrade next?

You are going to get a number of different opinions in response to your question, but in a situation like yours I usually advise people to upgrade their speakers first. Some people will advise starting at the "front end" with a new preamp or better source components, but if the speaker is the choke point you won't hear the improvements upstream. The Maggie 1.6's are a fine speaker, and are sufficiently revealing that they will reveal later improvements in your other components. After you upgrade your speakers, I think your very nice change should be to the preamp -- the Adcom GTP-500II was, at best, only an adequate preamp (certainly not up to the standard set of Adcom's GFP-565), and there are many better preamps on the market now for a reasonable price.