My understanding is that the transformers are the only significant change. The very latest ones are fitted with logic boards manufactured from a totally different colored material. This could be a change in suppliers, materials or an upgrade. The transformers are a big deal if you have electrostatics, and the factory charges ( I think) $1500.00 to upgrade to the new wide band version.
If you buy Wolcotts and want some tweaks, contact me. I have also done extensive tube testing with these amps for nearly a year, and have a fair amount of data on what works and how to achieve the sound you want.