yes, its all about system and preference matching.
here are my experiences based on the ones I have:
Sylnania 6SN7W (1940's short bottle)
characteristics: great tight bass slam, and mids are very immediate and slightly forward (giving you that sence of speed and presence). 2nd order harmonics is noticiable throughout the entire frequency range.
pros: very good for vocals, and other solo instrument performances where presence is important (like the cello of Yo Yo Ma). and very articulate overall. very rare and expensive.
cons: upper-midrange may sound "shouty" on bright systems. slight treble roll-off. Sylvanias, over time, have the tendency to turn microphonic, even for the VT231, JAN, and W specs.
RCA 5692 red base
characteristics: very quiet because its industrical grade. This is known to be the sweetest sounding 6sn7 (sweet meaning full of 2nd-order harmonics, and prolonged note decay). tonal balance is neutral
pros: very quiet. will last 15000 hours. the sweet sound will make your system sound as if you have sonus speakers. very good for classical music and large orchestral arrangements.
cons: bad for fast/dynamic music like dance and rock. will make treble notes sound sluggish as "essy" (parang prito ang tunog instead of metallic cymbals). will make mids sound too tamed. in other words, it will not rock!
ElectroHarmonix 6SN7 EH
characteristics: i call this the tube that sound like solid state. its very quiet, but lacks 2nd order harmonics. well, there is some harmonics is the bass region where bass is noticable "rounded" instead of being crisp.
pros: good for rock. quiet. inexpensive.
cons: tubeheads find it harsh-sounding because of the lack of harmonics.
HTH.
PS: To anyone selling Mullard ECC35, Tungsol, or Raytheon variants, please PM me!