As Highstream has mentioned above, I've tried a quite few reputable 6SN7 tubes on my new Cortese preamp for last 2 months (Melz 1578, Shuguang WE6SN7 Plus, Sylvania 6SN7W metal base, RCA vt-231 grey glass and the stock EH).
Among them, I like Melz 1578 most for their wide/3D soundstage and air of the music.
Also, I like the Shuguang tubes for their clean and organic sound.
For 6922 and E180F, Mullard tubes replaced them with very noticeable improvement. The Supratek Cortese sounded really good with the stock tubes, but once equipped with good NOS tubes, it sounds amazing to me.
Among them, I like Melz 1578 most for their wide/3D soundstage and air of the music.
Also, I like the Shuguang tubes for their clean and organic sound.
For 6922 and E180F, Mullard tubes replaced them with very noticeable improvement. The Supratek Cortese sounded really good with the stock tubes, but once equipped with good NOS tubes, it sounds amazing to me.