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said:

Another is that there is high magic in there, particularly the
transformers. The OPTs are probably cheaper to wind than the best
quality conventional ones of the day. Although the Mac patents have all
run out on their tube amp designs, there was even then Prior Art even
then which would have clearly got one of the key patents thrown out and
many claims of the two others minimized had someone wished to do so.

This isn't exactly true.
While the McIntosh patent of CFB seems an easy approach, manufacturing
said trannies certainly wasn't.

Ask Peter Walker (QUAD II) and Menno vd Veen (Plitron, Amplimo) as to
why.
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.

--
Sander de Waal
" SOA of a KT88? Sufficient. "