TUBE....Starting with Power or Preamp ?

A lot will depend on speakers you are using or want to use...
I would consider starting with a good tube integrated amp and go from there. According to your stated musical preferences I would go with a used Cary or AE or other better tube integrated whih has pre-amp out jacks so you can add a more powerful tube amp (or use the SS amp you already have when desired. The tube integrated will also elimninate the beed for matching interconnects and let you have the option of having both a tube system and the current SS system you already have.
If you really want to keep with seperates and are wanting to retain and use seperates you already have...go for the tube pre-amp first.

I would start with a tube preamp. For around $550 to $650 used, a Conrad Johson PV-10 is hard to beat. The PV-10AL is line stage only; the PV-10A has phono. There is also the original PV-10 (with no A or AL) which has phono also.

Supakit:

We can't really answer the "sound" questions as posed, exactly, because it's so very dependant on what gear you're actually using. Speaking VERY GENERALLY, though, the tube amp will give you the full "tube sound" right from the start, with either a ss or a tube pre. A tube pre may give you much of the smoothness and musicality of the tube treble and midrange, without sacrificing much detail and bass "punch", when mated with an appropriate ss amp. Neither is necessarily "the better approach" - it completely depends on what type of sound you want. Since you emphasize detail, I'd probably suggest the pre-amp first, but you'll find advocates for both approaches. Note that pre-amps generally use fewer and less expensive tubes, which can be a consideration when buying used.

The last question is a stumper - almost any tube pre or power amp is great for that kind of money ;) Right now, just on this site, there's a Dynaco ST-70 for $250 (a venerable amp with a bazillion upgrade options and a huge knowledge base available to consult); a Moscode 600 hybrid for $600 (I have a 300, which needs attention but still makes lovely music, in their day these were wonderful bargains and to my ear, still are); a couple Anthemn Amp 1s and Jolidas on the lower end, not to mention some QuickSilver Monoblocks in the $900s. For pre-amps, there's an ARC SP-14 for $900 (ARC makes good stuff, though I don't know the SP-14 at all); a Conrad Johnson PV-10a for $650 (nice unit with phono stage); and several QuickSilvers, other CJs; etc. Personally, I'd go with the PV-10a to start out, it's a reliable performer with a nice sound and a good resale (important when you want to upgrade!), but I look forward to the other answers you'll get as I know there are far more experienced tube-o-philes around. Good luck!

chas

Getting a tube amp and a passive preamp or attenuator is another way to go.

Supakit, I would suggest you follow Sugargrie's recommendation and try a good used tube linestage in your system. I have found that the linestage can have a greater impact on the overall sound of a system than most people realize. Amplifier choices are often highly speaker interdependent: there is a strong synergy that makes matching the two a sometimes challenging process for optimal performance. The linestage will be much less system interdependent and will allow you to experience the values that a good tube component can bring to your system. As others have recommended, the Conrad-Johnson PV-10AL is a very good choice to try.