Hevac
You said "do not use the dual f(r)eq option, AC motors do not like it, they can and will fail if used".
I have no experience of running the PS Audio units being discussed so this may be true of them but it is certainly not true of frequency controllers in general.
AC synch motors can usually be run at a wide range of speeds using frequency control if the controller is designed to achieve this and due respect is given to the heat load and mechanical aspects (eg bearings). A range of 50 - 150% of spec is considered conservative.
Of course if you use a single phase control then the value of the cap on the motor will be wrong for all but one frequency. This may be the cause of some of the failures you predict.
Mark Kelly