Take a peak inside to see how the design is and what parts are used. How is the power supply? My guess is that you will find while some are more robust that others, the overall parts are close to being the same so overall the sound differences are not so much better. That is why you have to learn what makes the a component sound the way it does. Once you understand that, you can then find components that are much better then the average manufactured products you are considering.
As for how a preamp colors the sound, well all preamps have their own sound characteristics. There are preamps that can do both but again you have to know what to look for in the design.