Preamp Tube Rolling

The 'input' tube, has a more discernable effect (usually the first tube in the circuit). Other tubes, such as cathode followers, have a much lesser impact, but they do have some so don't, ultimately, ignore them. If you don't know the tubes order ask the manufacturer of your pre-amp. Visual appearance of order may not be determinative, and in some its downright confusing.

And if your preamp is tube rectified, rolling your rectifier will also have an audible effect. I agree with Newbee that the first tube in the circuit will have the biggest effect on the sound, but my experience is that the rectifier will have a bit more effect than the "output" or subequent tubes.

David