I FINALLY got a realistic response from the developers tech support (after many months). I had to remove it and reinstall the recently updated installer for M-Tron Pro. Then launch Logic Pro in 32-bit mode, open the AU Manager, and see that M-Tron Pro was unchecked as not available. Check it, which allows me to force a re-evaluation of it by Logic. It was approved and works fine now. Quit, change Logic back to 64-bit mode, and then go back to work. The developer claims they are working on a 64-bit version of M-Tron Pro. Not sure why it is taking so long. I suspect they're developing for a future release, specifically. I'm crossing my fingers. Side note of interest; When I reinstalled M-Tron Pro and went into 32-bit mode and then into the AU Manger, something I've done several times previously, I was shocked at the number of plugins that suddenly appeared in the list, all being unable to load! Wow! Reinstalling M-Tron Pro triggered off something, obviously. Now I have to go through them one by one, and figure out what is what. Wow... At least this save six or more song projects that heavily relied on M-Tron Pro (and thus haven't been touched in over six months).