I liked the Mullards,and preferred the military 4004/8137s to the standard 12ax7s.They have greater extension on their top-end.
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if you want a "softer" sound you might be better off with a ge 5751 black plate from the 60's, or before that.
if your dac can accept any 12 volt tube, i have found the 5814 tube to be kind to the ear. i have an alpha dac and i use ge 5814 black plate.
if you want to go with mullard tubes, look for early mullard (1950's) long plate.