Sander:

If it was cheaper or easier, a company like Audio Research would
certainly use it in lieu of their partially cathode coupled circuit,
which has inherent disadvantages as opposed to CFB circuits.

Hint: it has to do with the physical limits of enamel copper wire.

Which is why there are other insulations. There's a remarkable paper
floating around by a guy who built a trifilar primary Mac amp before
Mac did (!) and he used a Formvar wire, which I'm sure has been
superceded since 1956. I can't remember the guy's name or I'd look it
up-i think he was from Cleveland.

Also I'm glad you said it--ARC isn't above rampant cost cutting
either. Indeed they are probably not as well built as the Macs.