Hi, I am also using VTL (750).
Perhaps you contact Albert Porter.
He once recommended in a forum for his VTL-750: "The 12AT7 makes a lot of difference, favorites are Amperex from the early 1960s, pinch waist Phillips and Telefunken diamond bottom."
User microstrip once replied to Albert...
Hi,
I am astonished that there is not much discussion about this loudspeaker.
I heard it in Munich at MOC in May this year and was very impressed. Very natural sound, wonderful soundstage, seamless driver integration - excellent on jazz, classic, piano and voices. Perhaps I would buy the speaker...