Brent Jessee is a great source ,
Tubemonger is another one , I replaced 3 RCA black plate 5751's
with " slovakian " 12ax7's ( ecc803s ) in my phono preamp , Love Them .
The gain of a 5751 is 70% of a 12ax7 but you don't hear 70% less ,
the difference is about 1.5 dbs or a notch on you volume control .