That preamp as well as TAD amps were made my a Chinese company called BEZ. Paul of TAD would then finished them out with some finishing steps.....
The transformer is most likely a BEZ and they still are in business and sell transformers. I bet you can take the old one out, take a picture and jot down the specs, then email BEZ for a replacement. I heavily modified a TAD 60 and have emailed BEZ thus this knowledge.
http://www.bezdz.com/engindex.html