@rauliruegas ,
materials is always important but I think that the more critical and important characteristics belongs to the rigth build transformer design and the quality level for the excecution of that design.
I agree. I tried to say that in the sentence about trusting the Lundahl brand more than the Cardas. You said it better.
But I don't agree about vintage excelling modern. As I understand it, amorphous core (Metglas) has only been understood and available for the last 25 years or so. Also, modern machines allow for more precise tensioning and placement than previously, hence, at the very least, more consistency in manufacture.
As for FR, 1931Ag's are 10-100,000 Hz =/- 1dB, which is more than adequate for a puppy's hearing, let alone mine; and I trust Lundahl's current testing and reporting more than vintage.
Finally, my testing shows the 1931Ag to be pretty close to pure gain. Not a whole lot of room for improvement there.