The output transformer "sticky" post on the G*S*G forum has a lot of information. I built mine with EDCOR transformers. EDCOR is a US company, and reliable. The downside, everything is made to order so it takes a few weeks to get your parts. The upside, is great prices, and service. I check EBAY once in a while. It is about time the sellers learned that "vintage" refers to wine, and not some dirty piece of junk they found in their grandpa's shop, or a flea market. There are some great sellers on EBAY to be sure, but seldom, great prices.
Check this out:
http://tubelab.com/articles/component-testing/output-transformers/.
This guy makes great output transformers:
https://www.electra-print.com/In general... bigger (more iron) is better, especially at the low (bass) end.
If it is not making X-rays, your B+ is too low.