Warm, yet detailed tube preamp

The Cary SLP98 is a very warm and smooth sounding unit, even moreso than the SLP05.  Audio Research and LTA are not particularly warm.

I suspect that you Zu speaker is the reason for wanting a warmer sound.  While I appreciate the liveliness of those speakers, the have a very hard edge to their sound that I don’t think can be easily tamed with other component choices; it is too extreme in sound for that—you live that sound or find something else.

hmn...van alstine fet valve CFR; T10RB is a little more tubey
 

Backert is probably the best pre for the money.  For those with inclined ears and less aggressive budgets, my people: Apt Holman is powerful good too .... no tubes however.

Honorable mention: Audible Illusions ... 

https://www.audibleillusions.com/

I go to plenty audio get togethers  and for quality and build in the $5-6k range Linear Tube Audio made in U.S.A designed by Tube guru David Berning 

then later upgraded still if you use xlr the Swedish Lundahl Transformer upgrade 

and order with the 6 sn7 tubes , then there is a small set of tubes 12at7 or others 

very good all around and not like many that use the cheap $20 silver can alps pot like prima luna , these guys you a true resistive ladder  for the volume. Which is ultra important for detail. Mike at Audio Archon  gives excellent service and a fair price off on retail.