I’d recommend a real separate preamp instead of the input buffers. I use a switcher switching between
1. Streamer -> ICEpower 1000ASP dual mono amp (B&W 802 speakers)
2. Streamer -> Audio Research LS27 tube preamp -> ICEpower amp
3. Streamer -> Pioneer SC-LX904 AVR
I don’t think I can tell the difference blind between all three and I can barely tell the difference if I knowingly switch between 1-3. I would need headphones to bring out the difference.
I believe class D is so fast that it mitigates the effects of input buffers or preamps.
I also don’t think paying up for various class D off the shelf modules for spec adds value. You would have to buy boutique class D amps like AGD or Laiv for value added differences.
btw, my preamp/tubes are neutral (my preference). I use a u-curve EQ from the streamer for my desired color. The tube preamp was a choice to add an additional layer to the sound. But honestly I'm not sure I can even hear the layering. I hear something slightly different but I can't identify what that something is, ie the layering or simply amplified tube noise. The edges might be slightly smoother but it's questionable. Also, the speakers produce measurable soundstage so I don't need psycho-acoustic soundstage from an amp or preamp.

