Do tubes last longer in Preamps than Amps?

I would suggest the CAT preamp for Krells. Performs perfectly with my Levinson 20.6 mono's. Only uses single-ended cabling, though. The line and phono stage tube compliment costs around $120.00 to $200.00. If you don't leave it on all day and night, the tubes should last more than 5 years before they start showing signs of age.

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.

The tubes in my BAT preamp (a really good preamp by the way) are rated at 5000 hours. the tubes in my vtl preamp lasted about 4 years before starting to fade. Also , replacement cost was under $100.00 for 5 tubes- much cheaper than the power tubes.