i did a Pro Jr project and put one of Graydon's (GDS) Heyboer OTs in it and wow! An amazing improvement in every department! It was phyiscally almost three times as large as the original Fender OT. In a nutshell: more volume, more tone, more dynamics, more harmonics.
FWIW, Heyboer OTs were original in many 'wrecks. I first tried them 15 years ago when everything had to be special ordered (they did not have a line of "canned" OTs). Always way overbuilt both in freq range and wattage handling compared to off the shelf type replacements.
Heyboer is one of the transformer companies I used/use for my amps. I think in general they make very good products and I don't hesitate to recommend them. However I leave myself open to using other manufacturers for certain designs--it's about whatever works best, not about brand-name loyalty.
Thanks for all the replies -- sounds like they're well regarded, plus, they're made in the enchanted mitten. I've only knowingly played an amp with a Heyboer output transformer and choke once when I tried out a Valvetech Hayseed -- sounded wonderful, like my old AC30TBX but with a boomier low end.