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As you can see, these JJ variants feature long plates. That's the rationale behind JJ providing different designations, which they have done with some of their other products. At the time they wanted folks to take notice of these new tubes, and their potential for different sound.
Maybe that's not completely obvious, but they offer a more different sound than the typical 12AX7 and 12AU7 offerings. In general, I've found that short plates produce a tighter, punchier, more focused sound and vice versa. That's not to say it relates to brightness or harshness, as the better variants stay far away from those attributes.
If you do want to experiment in the future, give the Shuguang (or Psvane) triple mica 12AX7 a listen. They're about $10, and compete with the very best ever. You'll hear quite a bit of difference, and begin to determine how to best steer your amplifier to your tastes
^^^ Actually, several months ago I decided to try the Shuguang 12AX7b silver dragon. I purchased two matched sets for line my stage and phono, requesting extra low noise tubes. I have used them quite a bit, but in my preamp/phono stage they are not as pleasant as the Psvane 12AX7-TII or Mullard old stock. Of course the latter are 5 times the price of the silver dragon. They will come in handy as backup. Did I purchase the wrong Shuguang or are you referring to the same 12AX7b?