Velvia

I want to increase the saturation more. How can I do this?

To increase the saturation even more, you may simply duplicate the layer and adjust the opacity of the new layer(s). Or, for better results, use larger values in the channel mixer. For example, instead of setting the Reds in the Red channel at 150%, try using 200%. Don’t forget to compensate the other colors with lower settings (ex. -100% blue; -100% green).

If I just want to increase saturation, which tool will produce the best results? The Channel Mixer or Hue/Saturation tool?

The Channel Mixer tool will produce better results. As an experiment, follow Velvia Photoshop tutorial and duplicate the Channel Mixer adjustment layer twice so that there are three Channel Mixer adjustment layers. Now duplicate the background layer, move the new layer to the top, and use the Hue/Saturation tool on the top layer and increase the saturation to +100. Hide and unhide the top layer repeatedly and notice the differences. You’ll notice that the layer you used the Hue/Saturation tool has a posterize effect.