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.
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.