Not in a Herron phono stage, but for what it may be worth, yes I have tried GE 5751 black plates in place of Telefunken, Siemens, Raytheon and other 12AX7’s in an EAR phono stage that I used to have. Good tube with a distinctively different character than the various 12AX7’s as a group. Needless to say each other 12AX7 had its own signature. The Tele’s being my favorites by far. The GE 5751 gave me a very tightly organized sound, somewhat “gray” tone with a little less sparkle up top. Reminded me of the way Shure phono cartridges sound to me.
This brings up something that I don’t think has been stressed enough in the otherwise great commentary above. It should not be assumed that a 6922 (or 7308, or…) will always be better than the “lowly” 6DJ8. Depends on the particular tube and the circuit that it will be used in. Based on my experience, I would take a good vintage Telefunken ECC88/6DJ8 over many other brand 6922’s and 7308’s that I have tried without hesitation.
Good luck.