I would like to thank everyone who responded, what a great forum! @TLG Guy was right on about the gasket, it was compressed to the point I could see it acted like glue.
After a good night sleep, I decided to give it another quick trial. I remove the binding post caps to exposed the holes, then I put a good size hex key through them, 2 holes at a time to ply the plate loose. On the 3rd ply it popped right out. By the way, I tried hammering it with a rubber mallet before and it didn't work no matter how hard I bang it. It might have helped the hex key step though.
I may be the only one who has such stupid problem but I thought it wouldn't hurt sharing the trick that worked in this case, without the risk of scratching the shiny black finish.