Which one produces a bigger change... changing a rectifier tube or tubes on my DAC?

Rectifier diodes, tube or otherwise, make a huge improvement. More detail yet more liquid, blacker backgrounds with improved dynamic shadings. Have never done that particular one but all the diode upgrades I have done were extremely cost effective.  

If however you are already using a rectifier tube then what you are really talking about is tube rolling and that is a different question altogether. I would not expect a huge difference between tubes. If you are talking about replacing SS output with tubes on the DAC then I would expect a bigger improvement there.

Do both for best sound get NOS Amperex not cheap but the best sound.

Yes.. Definitely tube rolling as I have already replaced the stock Modwright tubes. I was thinking the Amperex nos also.. have you experienced them to hear a difference? if so.. what did you notice?