2v, 4v, or 1v input better?

Yup yup.
The options are for equipment matching.
That iS the only absolute.
I have +- trims on all inputs.
That would probably short your circuit.

you need to know the max output of the source equipment. then go up a bit above that.

many cd players range 1-2v, so use 4v.

your phono stage is probable safely below 2, try 4 to be safe, then if not enough volume, try 2.

The setting that can achieve full power from the amp but not overdrive the input setting.

+1 - imhififan👂

It depends on the sources, cartridge and phono stage output voltage, play with those setting and decide what is best.