This is old info,
Still interesting to revise:
http://tubelab.com/articles/component-t ... nsformers/With anything like 6L6 El34 etc I would go for the higher spec 15 Tx.
But its a gamble on saturation. If you already have some output Tx's you could test them.
I run quite a lot of bass through my equipment on SE and use cathode FB AKA >>PYE.
Its only my opinion but a 10 watt Tx is more like EL84 or 6v6. But even then I use XSE15.
I guess it depends if you are going to run film explosions through it
Again the output winding spec is a pain where 8 ohm is better than 6 ohm with cathode FB.
It depends what rocks your boat in the old days people always say the equipment output Tx was under spec, but there were some pretty big tx's even in HMV record players using N78 output tubes.
You can probably get away with smaller on PP due to the flux cancel across the Tx, but SE

.
Years ago I used some old paper dielectric transformers I found in an old stereogram on some waste land. I tested them and they were OK.
There is a difference with the dielectric and you can hear it. I notice someone was making old school Tx's with paper insulation but I can't recall who at the moment.
So if its old school I assume you will use CC resistors<<its worth a test I think you might be surprised. And or CF.
Its interesting to note how old school had a "thing" for high meg ohm resistors

and multi strand flex wire.
Obviously its on low gain circuits.
Regards
M. Gregg