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An incredible amount of work went into the amp modeling. I spent three years refining the algorithms. Heck it took months just to figure out how to model an output transformer.
The Ultra is a great unit. The Axe-Fx II is better. I wish I could retrofit the new algorithms into the Ultra but they won't fit. My latest modeling algorithms simply need a more powerful platform and that's why I developed the II. I listened to the users and developed a product based on their feedback.
The hardware in the Axe-Fx II is really good. Still using all film caps on the I/O. The D/A converters are now buffered with instrumentation amps for improved S/N when doing the 4CM. The circuit board is controlled impedance with gold-plated pads. The layout is "partitioned" and uses EMI filtering between partitions which achieves an incredibly low noise floor. The self-noise is so low that I can barely measure it with my audio interface.
There is a ton of RAM and FLASH for future expansion. There's a 200K gate FPGA that offloads tasks from the processor. Etc., etc., etc.
The Axe-Fx II is the finest hardware I've ever created in my 25+ years as a designer and I've designed probably over 100 circuit boards in that time.