Tube preamp reliability vs SS?

Very good advice given above.  It really depends on the tube type, the vintage/manufacturer of the tube, and the particular circuit design.  When tubes are driven close to their maximum rating, they don't last long, but driven modestly, the same tube might last for an extremely long time. There are people who claim that their tabletop radios are on almost all day and the tubes have never been changed in 60 years or more of operation.  

Within a given type, there are some tubes noted for their extremely long life.  These are typically vintage tubes.  I am certainly counting on these claims as I have invested quite a bit to get Telefunken ECC803S tubes for my phonostage (12AX7-type of tube); these have a reputation for being "lifetime" tubes.  I am also running some very expensive 348 and 349 tubes in my amp; the amp is rated for very low output (even for these tube types), so I am hoping that the tubes will last for many years.

A lot depends on the circuit the tubes are run in.  I know that some designers run tubes quite hard, like the old Counterpoint gear, and tubes had to be replaced every year.  

I would suggest buying a replacement set of tubes to have on hand at all times.  Once in a while, switch in the replacement tubes, if the sound improves dramatically, it is time to re-tube.

In some respects, it is easier to live with a tube linestage or preamp than a solid state one.  If you are someone who turns off your solid state linestage, it actually takes much longer for a solid state linestage to be fully warmed up and sounding its best; most tube gear is pretty much fully warmed up in less than 10 minutes. The big convenience factor with solid state is that you can leave a linestage on all of the time.  

In terms of ultimate longevity, in some respects, tube gear has a longer life.  It is easy to refurbish and maintain classic tube gear for decades while a lot of solid state gear is MUCH harder to find replacement transistors, ICs and op amps.

For me, the choice would come down to sonic preference and the kind of amp that the linestage/preamp will be driving.  If the amp is solid state, I would be more inclined to stick with solid state for the linestage/preamp.