Preamp for a GaN-FET amp...

The future for audio amplification is GaN FET and other flavors of successor generation "digital" topologies. Class A and tube amplification generally will become even more of a niche product area as the generation that brought those technologies back, mainly to high fidelity, leaves the market, which one way or another will happen. Tubes may sound lovely, but enough people need to be buying them to make their manufacture worthwhile. The Chinese domestic tube HiFi market will probably become the mainstay for market demand, as long as their government permits inefficient electrical goods to be sold under whatever decarbonization policies have effect there. If the Chinese leave the tube production industry, tubes are done. No worry, some clever engineers will find a way to recreate the tube effects in a digital topology for those who can't do without. 

To the OP: Peachtree has a couple of GaN amps, at least three now, two of which are standalone amps and the third an integrated. You didn't say what your requirements were or a budget, but the integrated they sell is $3K and has 200W power.

You might look at the Pro-Ject Pre Box RS2 Digital preamp. It offers dual DAC, a headphone amp and a selectable tube output stage. It has an I2S interconnect for use with the ProJect CD transport, but will work fine with any streamer or CD player.