Preamp tubes or amp tubes will give more change?

Start with the preamp tubes. It's not that I think that small signal tubes have a bigger impact on sound, but, they are a lot cheaper when it comes to experimenting, particularly where you have to buy multiple pairs of output tubes.

Nsgarch,

That is a terrific, detailed, answer. I believe the Cronus is an integrated amp, so we agree that the small signal tubes come first.

I would add to what you say that a particular tube may work better in one place than another -- if the amp uses the same kind of tube for the input/splitter and the driver, try the new tube in different locations.

You can drive yourself crazy (or have a lot of fun, depending on your inclinations) trying the almost endless variety of older tubes, but, that is a really BIG subject area.