I understand that many modern power conditioners are basically a power amp that is driven by a 60 Hz source.
You are wrong if you think that ferroresonant power conditioners don't work under heavy loads. It is all a matter of scaling them up. Back in the day, we used 40 KW and larger ferroresonant power conditioners in mainframe computer centers.
If you want a constant voltage supply, use a motor driven variac.....also not found in 'power conditioners'.
No such thing in sight - note the power amp circuit at the top of the picture.