The reason for the UL transformer and 6AN8 tube is that Sunn amps started as Dynaco hi-fi amps with a custom preamp in a cabinet made by Conrad Sundholm in a garage in Oregon. (This was circa 1965 or so. Conrad's brother was Norm Sundholm, bass player for the Kingsmen of "Louie Louie" fame, and a bunch of other frat-rock classics, and he built the first one for him.)
They were either a Dynaco Mark III (UL 6550) or Dynaco Mark IV (UL EL34) in a head cab with their separate custom guitar/bass preamp. Later built on their own complete chassis, supposedly still buying power and output (UL) transformers from Dynaco. Eventually they added solid state add-on boards for tremelo & reverb to some models, and even later, went to all solid state amps, of course.
I have one of the early ones with a full Dynaco MK III amp inside. But if you want to see something REALLY weird, I also have an amp that is some kind of early Sunn copycat, called a "Polaris," which is conceptually similar: a Dynaco Mk. IV (UL EL34) with a custom preamp in an even more strangely shaped head...