We re moddeing my valveking and i saw that my speakers are 16 ohms but the slider knob on my valveking is set on 8 ohms.
Is that knob for the speakers or the tubes?
And should i set it on 16 ohm? Does it make a difference?

Is it a 2x12 combo? If so, and both speakers are 16 ohms, the total load provided by the speakers will 8 ohms, so you should leave it set to 8.
You cant set the imedance for the valves.
The impedance is for the speakers.

If you have 2 16 ohm speakers it'll be 2 16 ohm speakers in paralell, which is 8 ohms, so its allready on the right setting.

It makes mismatching your amps impedance with the speakers can strain the output transformer, (there are "safe-ish" mismatches though.)