I use a pair of Ei 6DJ8 (the regular, not the gold pin, not the elite) in the DAC section, and have used these for over a year. The sound is absolutely terrific, detailed with a good amount of air, deep tight bass. This was the tube recommended in Bound for Sound for the Monarchy M24 in the original review of the DAC. These Ei tubes were cheap (only about $5 tube) but I had some problems with noise with several tubes. I think it took 3 pairs before I found the pair I am currently using. I think something happened to the Ei factory a couple of years ago, and so I am not sure how available these are anymore (I bought mine from BOI Audioworks, who are currently out of the tube).
Other tubes recommended in issue 180 of Bound for Sound for the Monarchy DAC are: Sovtek ECC88WA and the Sylvania 6KN8. These particular Sovteks are pretty hard to find.
Mr. Sawdust, please post your findings with the Amperex as you experience them.