To be honest my expectations for this 12AX7 was low. I haven't had a JJ tube in any of my gear for well over a decade. Nothing personal, mind you, but back then I was all about more exotic tubes like the C3m, 5687s, EL156s, and whatnot. JJ has been around since 1994, built on the ashes of Tesla and Ei. Their small signal tubes aren't mentioned all that much on the forums I visit, or at least I missed the posts.
This particular version of the 12AX7 looks to be based on the famed Telefunken ECC803S, which also had a Tesla copy. The architecture has small plates that are very similar to a 6DJ8. This should translate to low microphonics.
As for the sound, at least as the gain tube for a power amplifier, the JJ ECC83S was better than I expected. There is an overall darkness, making this a good match for some zingier MC cartridges or digital front ends. This is a "chocolate" expression of the music, with solid and tuneful bass, warm mids, and a high end (to my older ears) that is even less detailed than a Mullard.
Inner detail with the JJ is less than the long plate Mullard; a slight smearing of instrument and vocal space along with soundstage depth. Nonetheless at least the JJ sounds like an vacuum tube unlike some of the inexpensive Russian offerings I've heard. So, much like the TJ and the Psvane, a solid offering but, and your mileage will vary, not exceeding the better old stock versions of the 12AX7 I've heard.