Any thoughts on permalloy output transformers?
Permalloy = added Nickel, not sure how much in their case.
For low level transformers such as microphone IP and line OP generally up to 80% Nickel is added.
The result is lower low frequency distortion and a cleaner overall presentation.
This may or may not be to your liking, for example many vintage 'clone' transformers such as Neve (mixing desks)
output transformers are preferred in all iron as it adds to the authentic vintage sound.
So in a nutshell it will be subjective.
cheers
Terry