Tubes that are refered to as "pre-amp" tubes last longer than tubes that are "pover-amp" tubes.
Whether or not the pre-amp tubes will last longer in a pre-amp or a power amp is another question, but in a power amp that uses both, the power tubes will need replacing before the pre-amp tubes will.
Whether or not you should leave the tubes running all the time is a matter of debate. Tubes wear from beiing on and heated, but also wear from being heated and cooled. This would depend on how often you turn them on and off and how the tubes are used in the particular circuit, but preamp tubes are much less expensive than power tubes, so for you part of that will be a matter of personal preference (you might choose to leave it on all the time so you are always "ready to go").
Hope this helps.
Whether or not the pre-amp tubes will last longer in a pre-amp or a power amp is another question, but in a power amp that uses both, the power tubes will need replacing before the pre-amp tubes will.
Whether or not you should leave the tubes running all the time is a matter of debate. Tubes wear from beiing on and heated, but also wear from being heated and cooled. This would depend on how often you turn them on and off and how the tubes are used in the particular circuit, but preamp tubes are much less expensive than power tubes, so for you part of that will be a matter of personal preference (you might choose to leave it on all the time so you are always "ready to go").
Hope this helps.