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.
I know this is an old question but I was just listening to a podcast with the founder/owner of Border Patrol Amps and they really focus on 300B amps. He said they pair bigger output transformers so you're never over about 30% max. There were more specifics but they escape me right now. Those amps cost a butt load and he's been selling them for years so I feel cool with his position.
The larger OT will normally have better bass response. The 10W Edcor do not have good low frequency response at all, the 15W ones are good and would probably work with a 300B. The other thing to look at is how many ma of current they are rated for. The 15W are rated for 100ma, which should be just enough for a 300B. The 25W are rated for 200ma, just gives a lot more headroom. And IMHO the larger OT just look better with a 300b tube.
I suspect a higher rated OT would provide more headroom than one rated just above the tubes output. Don't forget that the specifications are often based on constant test signals rather than the dynamic shifts in music.