The 12ax7 you have is a Shuguang very common Chinese tube. Now finding what kind of tube you want is the journey you must take. Read in the forum's here and audio asylum to get an idea of what you may want to try. Remember what you like is what you like! Here is the link to audio asylum http://www.audioasylum.com/audio/tubes/bbs.html.
Yes if you change the 6sn7 you need to change both. If one of the EL34 goes out. If it's not very old you could get away with just changing a matched pair. I wouldn't change only one. Other people may disagree? But EL34's are not too expensive you may want to try a match quad of a new brand. Again the EL34's are probably Shuguang.
Remember to enjoy the music. Some of us including myself have too many tubes and tend to roll them too much.
Yes if you change the 6sn7 you need to change both. If one of the EL34 goes out. If it's not very old you could get away with just changing a matched pair. I wouldn't change only one. Other people may disagree? But EL34's are not too expensive you may want to try a match quad of a new brand. Again the EL34's are probably Shuguang.
Remember to enjoy the music. Some of us including myself have too many tubes and tend to roll them too much.