In reply to andrew breckill:
To possibly add an element of confusion into things, I bought a Musical Fidelity X10d pre-amp second hand from Richer Sounds, which has a couple of valves in it, and found that it added a certain warmth to my Marantz set up (with an old NAD amp and some JPW speakers), and improved things generally.
It's been discontinued, but copies of the circuit boards themselves with the valves on are available from ebay to buy from Hong Kong and China.
I find it 'muddies' things just slightly now I've got a Rega CD player and amp, and some Spendor speakers, but I can attest to the warm pleasantness of valves. Different higher cost value set ups may not have muddied things slightly with my current set up. I would think lots of demoing to find the nicest set up for your budget might be the most helpful thing to do.
https://www.stereophile.com/tubepreamps/873/index.html
Edit: This is the thing, I may have confused tubes with valves, but either way, it's what one can buy the internals for from ebay.
Edit2: Ah, you're taking a punt online, if your set up is reasonably budget (no slur intended) it might be just the thing to make things nicer if not technically precise, and pleasure is where it's at in the end.
Edit3: Hong Kong might be the place to buy them, or carefully looking into reviews of the ones from China, I gather there can be culture of taking shortcuts in certain circumstances with products in China.
Post edited at 15:07