The answer is that the tube that the preamp is optimized for is the one that will sound the best (lowest distortion- smoothest sound with greatest detail).
The best sounding of tube of the list is the 6CG7 which is essentially a 6SN7 in a 9-pin miniature package. But its of no consequence if the preamp is designed for a different tube.
The best sounding of tube of the list is the 6CG7 which is essentially a 6SN7 in a 9-pin miniature package. But its of no consequence if the preamp is designed for a different tube.