@1markr
according to this review
http://www.enjoythemusic.com/magazine/equipment/0419/Audio_Mirror_Tubadour_III_Review.htm
" Tubes used in the DAC are two 5977 sub miniature triodes, a pair in each channel. They use a special tube adapter, which is plugged into a regular 9 pin tube socket. You could unplug the adapter, which would then allow you to do some tube rolling. So you could substitute tubes like, 6DJ8, ECC88, 6N1P, 6922 and the Soviet 6H23 P. After experimenting with many tubes the AM team found that the 5977 were the best sounding tubes for this application. Second best sounding tubes are the 6N1P."
according to this review
http://www.enjoythemusic.com/magazine/equipment/0419/Audio_Mirror_Tubadour_III_Review.htm
" Tubes used in the DAC are two 5977 sub miniature triodes, a pair in each channel. They use a special tube adapter, which is plugged into a regular 9 pin tube socket. You could unplug the adapter, which would then allow you to do some tube rolling. So you could substitute tubes like, 6DJ8, ECC88, 6N1P, 6922 and the Soviet 6H23 P. After experimenting with many tubes the AM team found that the 5977 were the best sounding tubes for this application. Second best sounding tubes are the 6N1P."