I have the GR Research Modded Dac 60 and it is a fine unit with the right tubes. I had a huge stash of 6dj8 family tubes. As nearly always the Amperex are favorites.
Amperex Orange globes- best top to bottom
Amperex BB- little warmer and more plump bass
Siemens chrome plates- great all around/ detailed and expansive but thinner than the Amperex
JAN Phillips- best bang for the buck- highs not as sweet as Amperex
Mullard - Toe tapping fun, warmer and little looser bass- Musical less detailed
NOS Ei- very good top to bottom but slightly less textured midrange
Ediswan- good all around but less spectacular than Amperex, Seimens, Mullard
Amperex Orange globes- best top to bottom
Amperex BB- little warmer and more plump bass
Siemens chrome plates- great all around/ detailed and expansive but thinner than the Amperex
JAN Phillips- best bang for the buck- highs not as sweet as Amperex
Mullard - Toe tapping fun, warmer and little looser bass- Musical less detailed
NOS Ei- very good top to bottom but slightly less textured midrange
Ediswan- good all around but less spectacular than Amperex, Seimens, Mullard