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

Trying to see which tube upgrade to do first on my Modwright Oppo UDP205. Tubes on the DAC or rectifier tube? At the moment Im using Sylvania on DAC and a 5R4-GY on rectifier from Andy at Vintage Tube. Any recommendations would be great. Thanks.
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?