Well I didn't do any scientific research, I only compared two OTS amps one, with Ceriatone transformers (my amp) and one with Made in SLO transformers... like used in these HiFi amps...
... and to be honest I couldn't say the difference was that big... all I can say, I could hear the difference but my amp was already burned in, the other one was brand new... and it was really impossible to say, that transformers were the ones which caused that difference.
Bottom line... both transformers passed my "personal" test and I wouldn't lose any sleep over this.