When the proper impedance is connected to the proper transformer tap, the load will reflect the proper to the power tubes, in the case of these output trannies, 4.3K. Connecting a lower impedance load to a higher tap will reflect lower than 4.3K Ohms, and connecting a higher impedance load to a lower tap will reflect higher than 4.3K Ohms.
So connecting a 12 Ohm speaker to the 16 Ohm tap would yield a bit more power but a bit more distortion as well. Connecting a 12 Ohm speaker to the 8 Ohm tap will provide a bit less power, but lower distortion.
Now an output transformer's winding ratio is not linear. So when you see a schematic with 4, 8, and 16 Ohms, and the 8 Ohm tap is at 50% (the center tap), that's wrong. The center tap is actually 4 Ohms. The 8 Ohm tap is actually at 70.7% of the winding. This is a result of taking the square root of 8/16, which is 0.707. For 4 Ohms it would be sqrt of 4/16 (.25) which is 0.5. Therefore in theory the proper tap for a 12 Ohm speaker would be sqrt(12/16) which is 86.6% of the secondary. So that puts it about 16% above the 8 Ohm tap and about 13.6% below the 16 Ohm tap. Not exactly between the two taps, but fairly close. Usually when this close the recommendation is "Try both and choose what sounds best."
I would start with the 16 Ohm tap for a couple reasons. First, it uses the entire secondary of the transformer, and second, the feedback loop is usually taken from the 16 Ohm tap so any speaker interaction occurs there and the feedback is applied from the speaker connection and not at a lower tap. Just my $0.02 worth.
Hope this helps,
Ron
