It would seem that while tubes have the preferred distortion ratios, their total distortion is higher than all solid state preamps. So, what is it that is creating this preference and is this something that solid state preamps can "copy"?
@snapsc I think this is the same issue that has kept tubes alive in the power amplifier arena for so long. By comparison, the solid state preamps I've heard sound less lively and thinner than most tube preamps I've heard. I think this relates to distortion but I've done no measurements one way or the other to confirm this. However, tube preamps tend to be much lower distortion than tube power amplifiers. Solid state preamps tend to be even lower distortion; the 'measurement guys' will have you believe that distortion is so low as to be inaudible.
Tests we've done suggest otherwise until the distortion is below the total range of human hearing, which is about 120dB (so less than -120dB). That is my surmise; a lot more research has to be done in this area and I don't see anyone actually stepping up; the measurement guys say this was all solved 40 years ago yet clearly, it wasn't.
The result of that is tube preamps from 40 or more years ago still sound musical if properly refurbished but solid state preamps from that long ago don't get taken seriously. So I this is the best answer I can give, which is not at all definitive; belief and knowledge being two different things.