White oak is common in the USA, at least, it used to be. At an earlier time, a lot of furniture was made with solid white oak that survives today. I don't know if white oak trees are still common, but I hope so.
Those mid woofers are Seas W16s.
I have that same model speaker, but finished in ropey cherry veneer instead of white oak. No wonder I like them
.
I also like how the plinths are finished with a similar veneer, instead of the usual black paint.
TLS Guy uses the very similar Seas W18 drivers in his speakers.