What are the more subtle sounding tube preamps?

OP--for cheap, the Schiit Freya is somewhere between tube and SS...actually both, and a passive. the tube section is subtle (as in not very "tubey" or lush), imo, particularly with the stock tubes, but can be warmed up some with some vintage tubes. it’s no ARC 5, but, for the money, I think it actually sounds pretty good.

You might look into the Linear tube audio MZ2S,
Most definitely splits the difference between SS and tube based preamps.

The Freya is cheap for good reason I sent mine back after 5 days, even with very good nos sylvania vt-231 tubes I found it very noisy with a highly compressed soundstage.I do have one of their Yggdrasil dac's and I like it.

Best of luck to you,

Kenny.

You might want to check out the PS Audio BHK Signature preamp.  It is a hybrid preamp, with input triodes and MOSFET outputs, and I think it splits the sonic signature accordingly, which might just meet your requirements.  You can roll tubes to great effect to voice your system, it does impart a nice taste of the tube magic, and it is strikingly transparent, without being "glary" or "over-defined".  At its price point, I'm guessing that it is a tough act to beat in the right system.  My 2 cents.

Sonic Frontiers Line 3.

The McIntosh C1100 is amazingly subtle.  It is dead quiet and just presents the music as it was intended.