Not a silver bullet for great preamplifier sound. In my Audible Illusions preamp, the switch from 6DJ8 Bugle Boys to the Matsushita brought the SQ down. These appear to be very well made tubes, but not for my AI pre, perhaps others?
You get used the sound and over the years is starts to degrade where you don’t notice, but there’s a spark missing.
When you change the tubes the spark reignites and it’s love all over again.