Reference preamp with laid back and relaxed highs

This suggestion maybe from left field,I personally really like the Cello preamps,they all come with balanced outputs but they are fischer,which may require a custom cable atleast on the output of the preamp.Down side they are hard to find in good condition,however the sound is fantastic,

Few that exhibit such a coloration in the upper middle range & treble incorporated 2A3 R-ing tube ( I just sold such a preamp to a friend of mine - very sweet. However, it needed some work & tube rolling to get the best out of it. I didn't want to bother. ) could be coincidence....It will be very difficult to find (100%) what you are looking for. When you are going to find the closest match , you could possibly placed it in the hands of trusted modder/technician to fine-tune it to your taste(or)needs.

Esoteric Audio Research 864 or maybe the newer 868, I owned the 864, with two tubes in the line stage 1 x 12au7, 1 x 12AX7.
Rolling the 12AU7 made it very versatile to "tune" to your system, but its transformer ins AND outs were never anything less than right in the zone, quite relaxed, not limpid and the highs were exceptionally smooth without reticence.
Price is a bargain anyway you look at it...plus you get the legendary EAR phono stage thrown in !!!
They show up here with some frequency, one just sold for under 2K....good luck

Tvad, I enjoy the Boulder 2010 for specific qualities that you are looking for. It is the most relaxed and neutral preamp I have ever heard. Good luck and enjoy the search.

I'll take the contrarian tact as usual, Grant, along the lines of something that I know you and I agree on. In this instance, I would look into solid state preamplifiers.

For one, I think this is a better combination than the conventional wisdom of tube preamplifier/solid state power amplifier. The better solid state preamplifiers are far more likely to be more laid back and relaxed in the upper mids and treble, whereas tubes can be described as more lively and forward - "engaging" in the best scenarios, and bright in the less good ones.

Solid state preamplifiers are also more likely to offer balanced operation. Of course, one thing to always check is whether a component is truly balanced, as opposed to simply offering XLR input/output jacks.

For what it's worth, trying to turn things down a notch or two in the upper mids/treble is often just as easily accomplished by attacking the room. Finally, the most usual suspect in my opinion/experience is the loudspeaker crossover, especially, in today's loudspeakers.