Just to clarify - many audio companies used transformers made by another company. Mercury Magnetics was/is a reputable and popular brand. Purchasing transformers for power supplies and output coupling would be a make/buy decision. In the quantities of high-end audio, it often makes sense to buy rather than make.
Things have changed since the 1990s when the Io was designed and those decisions were made. Mercury may have ceased production of the product; which is no fault of the buyer. Aesthetix has invested in the equipment to wind and pot the transformers. They also found the wire they wanted for the quality they expected. And, interestingly, Aesthetix devised techniques to shield the transformers from noise prevalent in homes that did not exist 25 years ago.
I am curious about the failure modes mentioned above - transformers don’t have moving parts or much in the way of parts that wear out or age. It is copper wire wrapped around a steel core, potted and sealed. The potting will eventually break down, that would take decades. Input voltage spikes can do damage, but probably more to the downstream circuits than the windings (unless it exceeds the rating of the insulator).
And some of the old to new swaps in this thread are made to old model Io's with malfunctioning transformers. No mention of this monumental "upgrade" on Aesthetix website ! You gotta love the internet !
