These are high-quality, tested, NOS tubes. Most are "white-box" RCA's. This can be confirmed by the "squished-octagon" tube designation near the top of the tube glass. This style of tube designation was was unique to RCA manufacture. Most are side oval, or horseshoe getters - of the type typical of RCA side getters on these tubes. The RCA tubes have a reputation for a neutral extension in lows and highs, but they really excel in the midrange. Of course, any differences among Millett tubes are very subtle - if any at all.
The 12FK6 presents the lowest gain in the MAX or Millett, having a basic tube gain of 7. This gain is typically a few points lower in the actual MAX or Millett circuit, and will be even lower with low impedance headphones. The 12FK6 may almost have too little gain to drive high impedance headphones. However, the value of 7 is given as a comparitive starting point only relative to other Millett tubes.
It's been said that the 12FK6 has the most "air" and extended highs of Millett-style tubes. However, it may be a little light on bass, depending on your particular setup. It and the 12AE6 are a good contrast to one another.
Each tube has been personally checked for defects, the glass cleaned, and the pins straightened. They are guaranteed to work in your MAX or Millett, or return them for replacement (shipping not included).