How Soon Do You Realize You Don't Like a Piece of Gear?

By a flat frequency response, I think you mean very close adherence to the standard RIAA curve, which is not flat. Close tolerances with respect to RIAA are not an option; it's a requirement.

FWIW, In my opinion, it's not so much a matter of tubes v transistors; it's more to do with current vs voltage drive. I say this because I own one fully ss phonolinepreamp and one that is mostly tubes, albeit with a hybrid input stage.  Both outperformed a high quality current driven phono stage in my system, to my ears of course.  The best voltage driven phono stages are now at a convergence where solid state and tubes ought not to be very different. Oddly, for me the current driven soiid state unit had some of the sterility I formerly associated with solid state, but no longer.  That was the difference for me.  It was also surprising that at full gain, the current driven unit added a bit of transistor hiss to the signal. Neither of the voltage driven units do that, even at the extremes of gain.  This is purely subjective, and based on one example of a "transimpedance" phono stage, albeit an expensive and highly regarded one.

It’s unlikely you’ll change your mind over time. But for completeness, wait 50 hours. Then you will know for sure.