Don't know how to get any info at all on Expressive Technologies SU-1 MC step up transformer. All I know is that it's supposed to be one of the best ever. And also very expensive. I don't know of anybody who ever found one for sale. Very very rare. If you are lucky enough to ever find one, and have enough money to pay for it, then you are probably resourceful enough to find the info.
If you have to look for another option, which is likely, then you might look at the Kondo Audio Note silver wound transformer.You already have the M.A. Cotter Mk2. These are considered to be the next step below the SU-1, and these can be found. And it is entirely possible that the Kondo and Cotter units may be as good for less money. The SU-1 is so rare, that it is legendary, and I really don't know anyone who has compared them A/B. Sometimes legends get blown out of proportion, when they are so hard to find.
If you have to look for another option, which is likely, then you might look at the Kondo Audio Note silver wound transformer.You already have the M.A. Cotter Mk2. These are considered to be the next step below the SU-1, and these can be found. And it is entirely possible that the Kondo and Cotter units may be as good for less money. The SU-1 is so rare, that it is legendary, and I really don't know anyone who has compared them A/B. Sometimes legends get blown out of proportion, when they are so hard to find.