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.

All tubes anywhere in the system can affect sonics. That said, you want to first 'roll' those tubes that affect sonics the most (and choose what candidate you like best) and then the other tube positions as they affect sonics in descending order of influence. This way you will be able to hear the subtler changes from the less influential tube(s) and make the choice(s) you like. Otherwise you will be lost.

In descending order of importance, the amp should be optimized first. And in the amp, it's generally the smaller gain multiplier tube(s) that have the biggest effect on sonics, simply because they process (amplify) the signal the most. Starting from the input, a typical tube amplifier has:
1. a splitter/inverter tube (usually a 12A(T/U/X)7, 6SN7, or other twin-triode tube that converts a single ended signal to a balanced signal for further processing. The next tube(s) are the aforementioned gain multipliers (often the same type as the splitter/inverter, but in any case, usually another twin-triode type.) It's better if these first two functions use tubes that are lo-noise, and have their two internal sections reasonably matched. Which brands/versions sound best to you is your call . . . .
2. The next tube (or tubeS, depending how many pairs of power tubes there are) are the driver tubes. These basically modulate the power tubes' grid voltage using the amplified signal from the gain multiplier tube(s). The power tubes' grids are like a gateway (valve) that regulates how much power/frequency the power tube(s) send to your speakers. These driver tubes are next most influential (usually) on the sonic character.
3. Last in sonic importance are the power tubes themselves. The name of the game with power tubes is of course, Power. And in this respect, they influence sonics too, but in a slightly different way (than the 'color' of the sound), especially in the bass frequencies, depending on a number of factors; the most important of which are transconductance (gain or amplification capability) and maximum plate current output. These affect how good a 'grip' the power tube has on a low frequiency driver.

Preamps are another matter altogether, and can be addressed after you have the amp/speaker combo sounding the way you want. Preamps are, in their simplest form, source selectors, channel balance trimmers, and signal output attenuators (volume controllers.) Tubes in preamps basically need to be QUIET above all else. They don't do much amplifying (sometimes none) unless they are microphone or phono cartridge preamps, but they DO buffer (mediate) the various sources' output characteristics, in order to provide a proper output impedance for driving the amplifier. Bottom line? Preamps are 'traffic directors' in my opinion, and whether tube or solid state, should be QUIET and NEUTRAL. With respect to the signal, they should essentially disappear and have no effect on the sonic quality of the signal if they are doing their job.
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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.

In most all-tube integrated amps, the splitter/inverter tube (and often the gain multiplier tube(s) as well) are eliminated since the preamp section is wired directly into the amp section. So what you will usually see (at minimum) are a couple of preamp section tubes, a couple of driver tubes and then the power tubes.

The Cronus appears to be set up this way, except it has the one extra 12AU7 which (I'm guessing) is for the phono preamplifier section, a nice feature.
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all tube circuits are not created equal. i have auditioned tube components which don't responsd to tube changes. thus, there is no way to predict which component will be morre affected by tube changes.

i don't agree that small signal tubes have more of an affect upon the sound of an amplifier. i have noted , using both vtl 120 amp, consonance amp and mapletree amp, that changing the output tubes have as much or more of an impact on the sound than changing the driver tubes.

i don't agree that small signal tubes have more of an affect upon the sound of an amplifier. i have noted , using both vtl 120 amp, consonance amp and mapletree amp, that changing the output tubes have as much or more of an impact on the sound than changing the driver tubes.

First, I said the *gain multiplier tubes*, not "small signal tubes" or "driver tubes."
Second, did you do a controlled experiment with each of those three amps - comparing the effects of changing one group of tubes at a time (gain, driver, power) while holding the other two groups constant?
I stand by my statement; and further, point to a basic electonic reality, which is that where the signal undergoes the most processing (the 'gain' stage, in the case of amplifiers) is where it stands to be most affected by the hardware. YMMV ;-)
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Thanks all for sharing your experiences. I will try some input tubes first and output tubes later.

Thanks Nsgarch for your detailed post for me to understand more on the structure of tube amp.