I am so disappointed that Mag Comp closed their doors. I thought their Classic Tone trannies were excellent and about half the price of other brands. Kind of like the loss of an old friend. If I built amps for a living I would be at a loss without them. Maybe the one and only reason I would have traveled to Chicago (bang bang you're dead) would have been to tour their shop while they were still winding transformers.
I know several boutique amp builders use Heyboer. Some familiar names say they have their own "specially" wound transformers re-badged from Heyboer but that may be sales hype?
I found out the hard way that in the early years of the Egnater, made in China amps, they had a long stretch, perhaps one or two yrs, when their Chinese power transformers were better suited as Screaming Kitty Chasers for July 4th.
I know from watching and listening that Hammond has a rock solid reputation.
I do not know who Fender and Marshall source from? They may even make their own just to curb supply shortages and improve quality control.
I scoped out Mercury Mag apprx 4 yrs ago, bought one of their small single ended Champ style OTs, was not impressed for the high price and never went back.
It appears that Antek is making a well-funded play for transformer market share. I know they import (China I believe) and they offer a wide variety of power transformers. I do not know if they sell OTs? I bought a PT from them recently. It was half the price of other current products but I found out afterwards (my own fault) it made the correct voltages but was too weak to power my amp. I thought the workmanship, packaging and lack of hookup diagram was an indication that I wouldn't use one for a serious new amp build.