What are the more subtle sounding tube preamps?

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.

I have the VTL 2.5, the bargain model if there is such a VTL.  I have tried it paired it with VTL MB450 monoblocks, a McIntosh MC500 and an Adcom 5500.  The sound is very neutral.  The sound through all of the amps was marvelous, although the MB450s, which are Series I,  to me were the best by far.  This is the combination I use for attentive listening sessions.  The Mac seems to smooth the sound as if there is a light viscous coating on everything.  For background music I think I like the Mac the best.   The Adcom, although very neutral just didn't light my fire.  I am a true VTL believer and lust for a 7.5 preamp and Siegfried power amps.  My 450s are ancient and still have the original tubes.  To me they are sonically perfect in Triode mode and have proven to be bullet proof.  I highly recommend the VTL 2.5 and 7.5.