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?  

dac

What don't you like about the sound of your dac? I would recommend rolling the signal tubes before a rectifier, so what type of sound do you seek?

Changing a rectifier tube is like a tweak. You have to be pleased with the overall sonics of the component/system first.

@deanshias, I have a modwright Sony HapZ1ES which shares which your Oppo the 6922 based output.  It is hard to answer your questions because both the rectifier and the 6922s can make a significant impact on the sound of the unit.   It really comes down to tube selection. 

This is the best rectifier I have tried so far in the PS 9 power supply. 
https://sophiaelectric.com/collections/274b-5u4g/products/pages-se-aqua-274b

Over the years I've tried a large number of rectifier tubes including the one you are using and that Sophia blue glass 274B (which in this case is made to be a drop in replacement for 5U4) beats them all by a large margin.  Next best was the Canadian Westinghouse 5U4 which is now unobtainable.  I agree with lowrider57 in general about rolling rectifiers, but this Sophia tube is different.  It is not a tweak.  The Sophia tubes are expensive and worth every penny in my estimation.  

My preference for the 6922 tubes is the 60's halo getter Mazda 7308 tube.  It beats the well regarded NOS Telefunkens in my system.  I got those from Brent Jesse.  Brent describes the Mazdas as similar to the NOS Amperex but with better dynamics.  If you try the Mazdas, be aware that they have a definite burn in period.  Don't make any judgments on them until they have 50-100 hours.  Mine sounded pretty flat at first and I almost sent them back, then they just opened up beautifully.   Gorgeous, glorious sound in my system!

Which produce a bigger change? - both, but how much of a change depends on the specific tube.  I don't have experience with the Modwright Oppo but I do with my Lampi Atlantic TRP.  The best rectifier I have used so far is the Psvane ACME 274b.  It added substantial weight and saturation to the sound.  Signal tubes in my DAC tend to impact the soundstage (size and holographic quality) the most.  The Lampi uses power tubes instead of small signal tubes so my DAC tube experience won't be relevant but I can say my favorite 6922 for my VAC linestage is 1959-1961 NOS Amperex Bugle Boy.  Unfortunately this is a trial-and-error process but you can get some traction and eliminate some horrible fits by asking for this sort input.  Enjoy!.  

@brownsfan   +1

 It really comes down to tube selection.