Rolling Mullard 6DJ8 into GE 5670 Application

Depends upon what the tube is doing in the DAC.  If it is supplying gain, then yes you'll hear "a difference". What sort of difference is impossible to say without knowing the circuit and even then it would be a guess.  If the tube is just acting as a cathode follower that drives the downstream linestage or amplifier, then the difference afforded by different tubes would be much less profound or maybe not noticeable.  Are you sure the 6DJ8 is electrically compatible with a WE396A and/ or a 5670?  I am totally unfamiliar with the latter two tube types.  There's more to tube replacements than just having a correct adapter.

Are you sure the 6DJ8 is electrically compatible with a WE396A and/ or a 5670? 

https://www.radiomuseum.org/tubes/tube_5670.html

5670, premium model of 2C51, was introduced in 1946 and its successor was 6922 but it is not entirely the same tube. Some people though claim it sounds better than a 6922.

The issue for me is not how either tube sounds, but whether or not the two tubes are comparable in terms of their transconductance, mu, and plate resistance, such that you could plug one into a circuit designed for the other and still get the most out of the substituted tube.

I would ask the manufacturer or the distributor before I would ask here. I recently considered a different brand tube DAC that ran the same tube. I searched on line and saw the adapter and the recommended 5670 replacement tube. In my situation the E88CC was not compatible nor was the 6DJ8. Also in rolling that value tube ( I have a significant stash of nos ), the Mullard was dark and veiled and not to my liking. I realize YMMV , but I had better results with Tekefunken, Siemens CCA, Tungsram, and Tesla. Even the better Russian tubes beat the Mullard. I have dabbled with Rogue gear and they recommend Mullard, but it’s generally used to tame a harsh sound. Although the premium NOS Mullards are well received. But I’d really verify compatibility as I guess it’s not. Also in searching that topic the Westinghouse did seem to be the most poplar substitute. And the price is reasonable. Finding and buying premium medical grade Telefunkens is expensive and ugly these days. I worship my black coated G73-R’s . And I really do like Mullards for a rectifier or power tube. Happy Listening, Mike B. 

Prior to posting I did reach out directly to Maverick Audio and also Andrew at Sparkos Labs since I put his op amps in it.

When I purchased the DAC it has the GE JAN 5670 tube (upgrade from Maverick), then I rolled in the Western Electric 396A that some recommended as an upgrade along with the Sparkos Labs op amps that were a significant upgrade.  I didn't hear much if any difference with the switch from GE to WE tubes.  I think the tube is a buffer in the DAC, so it's very possible that the Mullard would yield little difference as well.

I do have Focal speakers, so some would consider them to be bright/harsh which may be why the Mullard sound great in my Pathos.  The person that recommended and sold them to me was very clear that his preference is very warm and the Mullard were the warmest he could find.  When I made the switch the improvement sort of defied logic in that it was a warmer sound, but also more detailed and just an improvement in every way over the hodgepodge that I had before.  I've briefly tried some of the "stock" tubes (can't think of the name off hand) and they were awful and came back out almost immediately.