@n80 As a person with a tube preamp and a SS amp, I'll share my 2 cents.
- Many prefer to have both a tube preamp and a tube amp. You get the full effect of the tube sound that way (depending on the equipment as some are more "tubey" than others). However, tubes in a power amp wear much more quickly than preamp tubes and tend to fail more catastrophically than preamp tubes - possibly taking out resistors in the process. You should be able to get between 5,000 and 10,000 hours from preamp tubes (again, depending on the equipment).
- Preamp tubes tend to get noisy before outright failure. Right now I have a tube in the left channel of my preamp (BAT VK-3i) that sputters a little as it's warming up. I've got a new set but haven't pulled the preamp out of the rack to replace them yet. I looked when I ordered the replacements and the last set I bought was in 2012 but I'm not sure of the hours. Of course if you know someone with a tube tester you could get them to check your tubes. :)
- In general, different preamp tubes DO change the sound but not as much as tubes in power amps. Preamp tube rolling (SS amp or not) is definitely a thing with some people (who have more money than me!).
- I would say that, given your description of yourself, it's not worthwhile to just swap tubes. However, when it's time to replace those tubes, check with folks here to identify the current helpful/honest tube sellers out there and consult with one or more of them. If you explain what you're after soundwise, my experience is that they will be more than happy to make recommendations within your budget.