'The term 'ultra-linear' was used to describe the invention that is the subject matter of US patent number 2710312A. You'll see it used in the text of the patent. Its an accurate description of the time about a break thru in amplifier technology. Diagrams of how it works are at the link.
It can be made switchable by having a switch connect the screen of the power tube to its plate as opposed to the tap on the output transformer.T
To do it properly, the output transformer would have to have taps for triode operation since the optimal plate to plate load for a power pentode wired in triode is a lower impedance than that used for UL operation. That could make the transformer a lot more expensive and the switching a bit more complex.
If the UL taps are set up properly, the amp should have less distortion in UL mode. FWIW, this was well understood in the 1950s when UL was first patented. Distortion obscures detail; so if the output transformers are properly designed it should be no surprise that the UL mode sounds better.
A secondary aspect of this is power tubes in UL mode are easier to drive than the same power tubes in triode mode. So the amp very likely is also making less distortion on this account too.'
from one of the previous discussion on the subject, all written by R.K. from Atmasphere