Though maybe not what I would call dark, the best 12au7 I have come across is the Philips Miniwatt 7316 variety. Beautifully detailed without tipping towards the slightly harder edgier Telefunkens or oppositely to the mushy Mullard. Most others I tried fell in between. The 7316 seemed to have strengths of all others I tried without the detriments. They aren't the easiest to find and aren't cheap for a good pair but they are by far the best IMHO.
