I have a Line 3 and replaced the back two tubes with early 60's Siemens CCa's - a very noticable improvement in sound quality.
Then I got to thinking what the other positions would sound like with NOS tubes instead of the stock Sovtek's (have read posts that it does not make any difference). I replaced the remaining eight with mid 1960's Phillips 7308's, I was once again WOWED with the improvement in sound quality.
So it seems to me that the better the tube in any position...the better the sound quality.
My two cents worth
Then I got to thinking what the other positions would sound like with NOS tubes instead of the stock Sovtek's (have read posts that it does not make any difference). I replaced the remaining eight with mid 1960's Phillips 7308's, I was once again WOWED with the improvement in sound quality.
So it seems to me that the better the tube in any position...the better the sound quality.
My two cents worth