Are your G12-65s 16 ohm speakers? If so, when they are wired in parallel (preferred) then you want to be set on 8 ohms. If they are 8 ohm speakers, you would have your amp set on 16 ohms if they are wired in series or 4 ohms if they are wired in parallel. Parallel wiring is more reliable.   

Oops Sorry about the lecture. I would be surprised if you heard a whole lot of difference. I believe that the different impedance used for speakers is to allow them to be used in different combinations with different OTs. For the same speaker and proper impedance tap on the output transformer I would be surprised if there is a really useful difference. I have tried my OTS with a 4 ohm 4 x 10 speaker combo and 8 ohm 2 x12 combo and a 16 ohm 15" speaker. They all sound different, but pretty good. With the OTS I am using an Avatar open back cabinet with (2) 16 ohm 12" speakers wired in parallel for 8 ohms. The speakers are Celestion Vintage 30 and G12H30. This cab actually out performs, in all ways, both 4 x 10 combo's that I have (one with Webers and the other with Kendrick Black Frames, both are top of the line after market speakers).

Just thinking here...
BTW this is just a theory, I did not test this, I'm just speculating here.

An 8 ohm speaker has a coil with 8 ohm worth of copper wire wound around that coil and 16 ohm speakers should have double length of same copper wire. OK if that's true, that would make that coil twice as heavy and that would probably have an effect to a final sound of the speaker, IMHO.
All this is crap, if they use different type of wire. So the length/mass of the wire is the same with 16 ohm speaker as with 8 ohm speaker.

Do we have anybody here, which would know that.