bdickens
I can confirm that Lexicon Pantheon has no GUI in Mixbus or REAPER , either.
I used to use Pantheon in REAPER and it had the GUI. It might have been a really old version because it was 32 bit only and when I went pure 64-bit at the beginning of this year, I retired the Pantheon and bought the Lexicon MPX Native Reverb, because I love the Lexicon algorithms, but wanted pure 64-bit with no bridging.
I notice that Sweetwater has it again for $49.97 which is what I bought it for last year, but be aware that this requires an iLok 2 or higher, which I don't mind but some folks might.
https://www.sweetwater.co.store/detail/MPXNatRev-e EDIT:
Also, I ran the two side by side and dialed my own patches to match some of the presets from the Pantheon. They are not identical in their factory presets. That said, I recently had someone comment on the acoustic drum sound I have on one of my songs. They asked me how I was getting that great room sound, or was I actually getting the sound from my drum room. It was the Lexicon, and the very reason I love the room algos they employ.