You could look out for some Mullard CV181 that I got from Audio Antiquary in Bologna, Italy.
Before them I was running the Psvane CV181T mk II. Those were good, very quiet, low microphonics, but the sound was competent rather than fully engaging.
With the Mullards (made in Blackburn, UK '68) the sound came alive, with much better bass extension, and unheard fine details. Incredible fine details that could still be heard despite the music playing loudly.
They are quite microphonic so you will need to protect them well from vibration.
Until I used them I wasn't really understanding why tube people tear their hair out over vibration isolation. Now I know why!
But the sound is totally worth it.
They cost an arm and a leg but they come in perfectly matched pairs - matched not just in gain but other crucial parameters too.
I have read (but I can't remember where) that with 6SN7/300B amps you are listening to the 6SN7 as much as the 300B. People throw serious money at the 300B, but this experience shows that you must not forget those little driver tubes!