The Fine Things are Always Hand Made
This is an old post, but just for the record:
Antek toroid, 120/120 to 12/12V. 10:1 ratio for 100:1 impedance. Series the secondaries for 40 Ohms. Series the primary for the center tap. This one works very well: https://www.antekinc.com/an-0225-25va-25v-transformer/ Unfortunately, they are out of stock. I have used it before and sound was as good as factory best my ears could tell.
Some people will tell you that a torroid is no good for an output transformer, but that is not true. You are looking at about 320 Henries for the primary which will give excellent low frequency response up to saturation at about 50 watts. Torroid transformers have very good coupling so high frequency response is excellent also.
So long as you have balanced DC current, there is no problem using a toroid output transformer. Unbalanced DC current will quickly saturate the core so you will want to avoid use of tubes in a manner producing unbalanced DC current. Use of a matched quad or two matched pairs of output tubes is not necessary but avoid two low emissions tubes on one side and two high emission tubes on the other.