The 300B tube datasheet puts its max output around 8 watts. Why would we want to use a 25 watt rated output transformer for this tube (operated SE)? The 25 watt OTs are so much bigger and heavier than the 10 or 15 watt versions, and most often I'm not listening at full volume anyway.
I understand that the original design used the 25watt OTs that were left over from a previous build -- that makes sense. But in a new build, I'd probably spec the 10 or 15watt Edcor OT.
There are several factors (conditions) that come together in your output transformer to give you an outcome.
I did stumble across a paper that is far more eloquent than I will ever be, but can't find it now.
First factor to consider is current on the primary side. Too light here is not a long lived transformer.
Next is frequency response.
Next is physical mass of the transformer. The overall mass does supply a certain amount of damping to the point of over damping in a very much large transformer.
Sometimes a 300b se is supposed to simply sound a certain way which you can only get with an oversized transformer.
To me, it all seems to be a bunch of shooting in the dark, hoping for the best result.
I do not think a 25w transformer is wasted in a 300b se.