$20k(ish) tube preamps

Atma-Sphere MP-1.

At your budget, you can add the Caddock resistors, and just about all the 'upgrades'.

Though Ralph Karsten is the manufacturer, I would either call or PM him (atmasphere), to get his take on this. He knows just about all that is available, and can give you accurate information, sans bias.

Bob

Call Tom Vu at Triangle Art Audio.  The L200 is really good.  

after a certain price - there are almost no improvements ... (only a case made of gold and ivory).

buy yourself a CAT SL -1 and you will be happy ... give the rest of the money to the poor (buy them new things, education or medicine) ... otherwise there will be no happiness ...
It is best to do this secretly and not tell anyone.

From my point of view (with my tastes)I would look at anAudio Research Reference 6SE. My system is all ARC Reference components… I have never owned or heard a more pleasing detailed and natural sounding system.

Convergent Audio Technology ( CAT )Black Path, or a used older model like the Renaissance.