newbie question on TUBE gears.

so far I've been using only SS gears, and thinking about a tube gear for the next upgrade..

I have a few question...

1. Is it ok to mix and match SS preamp and tube poweramp?
(thinking about Audio Research LS-12 which is SS preamp and maybe a tube poweramp for the next round of upgrades)

2. How often do you need to replace the tubes?

3. How easy/hard to get extra tubes and the cost?
(do you usually get it from a dealer? or directly order from manufacturer? avaible to order online?)

4. How easy/hard to replace the tubes? Is it easy enough to DIY or do you need to take it to dealers?

Any other questions I'm missing?

Thanks.