I read a lengthy comparison written years ago about 12AX7s and the writer felt that the 5751 gave a more engaging presentation.
The 5751 is a lower gain, quieter version of the 12AX7. So I tried a Sylvania black triple mica NOS and I agree with him. I find I lose interest in the music easily, even with a $275 12AX7, compared with the 5751. With the 5751 in my 300B integrated amp, I can’t stop listening. Have to force myself to get up and walk away.
And I don’t notice any difference in gain.