I've owned both amps you mention, I liked the Adcom GFA 5500 better. It is a newer design with much better sound than the GFA 555 series (or the Parasound, IMO).
I also owned the Ushers you mention. I believe the tweeter from the Usher BE-718 is a physical & electrical match for your tweeter. Find an owner of the BE-718 who has upgraded to the DMD tweeter, and you'll have a significant drop in tweeter upgrade.
You will have to negotiate a fair price, but the BE-718 owner will have the tweeters sitting in the box.....unless they've used them for a project speaker.