The 8Ω speaker with a 6V6 will use the yellow wire from the 125ESE.
The 8Ω speaker with a 6L6/EL34 will use the white wire from the 125ESE.

So using the impedance switch wired as the book (2005 edition) and the diagram on the site wiki shows, you will be able to switch between 6L6/EL34 and 6V6. Actually, in the 6L6 position, you can also use other tubes. I’ve used KT66 (my favorite), KT77, 6550s, etc. It’s great to play around with them.