Yes, I heard them both - very comparable and yes, the Mephisto was/is very reminiscent of the Antileon as you say. It's as though everything in the Antileon had been improved upon - noise floor in particular. The Mephisto is dead silent. In so many ways the Pandora/Mephisto are a throwback to the Sonata Allegro/Antileon - but just better. Gryphon has been doing their homework.
In the end, I sensed a "euphony"(?) in the Mephisto that the Colosseum didn't have - it just appeals to my ears. Someone else...maybe not. But it's important to note that we're talking SMALL amounts here. It's very likely that one could tune/attenuate these differences with their room, accompanying gear, etc. Ultimately however, the deal maker was $30K difference, which I then moved to the speaker portion of my budget.
All that said, the Colosseum is a world-class performer and the most stylish piece of gear I've ever set eyes upon.
Finally, I wouldn't look to compare the Neolith to the Altair - apples and oranges IMHO. The Neolith just struck me as a very interesting looking product.
Thank you...most interesting indeed. You are one of the few who has compared both.
By the way, I forgot to answer you about the Zanden 2500S...I have never had the chance to hear it. I have spent time with a few different Zanden 2000P/5000S series (Hong Kong, UK and Amsterdam)...but never heard the 2500S. I think they are different presentations though 'of the same family'...however, given that you are coming from Stahl-Tek, I would strongly say the 4-box, since its reputation is being far clearer, more delineated and detailed while also having an overall more solid presentation.