OK, I now have about 15 different tubes for the Ember and thought it was time to give some feedback to this thread. Work is really busy and my daughter is getting married this weekend so no time for a detailed multi-tube review right now so instead I thought I'd post a bit about my favorite tube so far.
Sylvania Gold Brand 6201
Mazda 4-Star 12AT7WA (a really nice tube that might have won if it was a bit lower gain)
Westinghouse Carbon Plate 12AU7A
Learned about these here: http://www.tubemaze.info/westinghouse-12au7-carbon-plates/ . Found a good deal on a pair on ebay with the Conn label.
This tube is a bit toward the warm side without sacrificing detail.
Nice deep bass extension, tone and impact. No mid-bass hump or bleed into the mids. Good texture and detail. Doesn't add bass that isn't in the recording, however it does bring out all of the bass in a recording.
Rich and sweet, without sacrificing texture and detail. Excellent for strings and gives realism to the piano beyond my other tubes. Male and female vocals are clear, forward and.well, rich and sweet. The combination of warmth and detail makes for some mice guitar distortion, too.
While a couple of my other warmer tubes suffer from considerably rolled off treble, it's not a problem in this case. While the highs don't have the "wow" in your face nature of the Mazda, if you listen you find they extend nicely and the tube delivers considerable high-end detail. One added bonus is the almost complete lack of sibilance. Cymbals are very clear and natural sounding.
Wide, tall and deep.
Open, airy, and very realistic. Good instrument placement and separation. Room resonances are impressive on well done live recordings (i.e., Amber Rubarth's Sessions from the 17th Ward).
FLAC > Foobar > ODAC or Uber Bifrost > 1964 Ears V6-Stage (also, Beyer T90 and Earsonics SM64)
Note: the V6-Stage are soundstage and detail monsters and it's nice to find a tube that let's the Ember drive them to their best. Also, it's my opinion the solid state section keeps the Ember very fast and prevents warm tubes such as the Westinghouse from rendering the sound as slow or mushy, which serves to preserve details (low to high).