The phono preamplifer stage must do two things at once.
It must boost the low voltage level of a MM type cartridge, which will put out a voltage in the 5 millivolt range.
Or if using an MC type cart, which will have an even lower output level, in the approx. 0.3 millivolt, to 1 millivolt range.
This is why some of the higher end preamps will have a switch to switch from one type or the other.
They switch the internal network to boost the signal even more, plus they use a slightly different load in the circuit due to the MC needs.
Most turntables came equipped with MM type carts.
And the preamp must invert/reverse the RIAA equalization curve that is used to make records.
So there is some electronics magic that must be done to make this work correctly.
Back in the 70’s a mathematician named S. Litpshitz showed that many of the Phono preamp networks used at the time to invert/reverse the RIAA curve were using incorrect network equations!
Which gave less that accurate flatness curves.
Add to that, the fact that consumer grade equipment manufactures used cheap standard 5% resistors and maybe 10-20% grade caps, also those old (60’s, 70’s) transistors likely have a higher noise spec. than today’s transistors in their input stage networks, and you get less than perfect RIAA flat responses, due the variation in the components.
Todays phono preamps should have been designed AND simulated on software to see if they return a flat RIAA curve.
Picking tighter spec’s on the Resistors and Caps is also a requirement to get the theoretical response.
So a preamp can have added ‘color’ due to an inaccurate inverse/reversed RIAA curve.
This will be due to several factors, such as bad RIAA network design, poor matching on the R’s and C’s, and noisy transistors.
Or due to those still not understood electrical effects (some would say magical) that people say adds color.
The basic question is always the same in my mind.
Which is: Can you hear a difference?
If yes, then which do you prefer?