The quality of the parts is important to the sound.
Cheap resistors will add noise (distortion) to the signal. Very expensive resisters will add a lot less noise.
Capacitors hold and release the signal, causing the signal to be smeared (distortion). This would be heard as lack of detail.
More expensive caps release the signal much quicker, thereby reducing the smearing and giving you better detail.
While you might not notice that there is distortion in the signal, if you could magically instantly replace all the cheap parts
with really good parts you would notice the difference.
Here is a simple real life example:
I have a Moscode 300 power amp. After having it repaired and getting slight mods from George Kaye, the designer,
I cheap bridge rectifier and replaced it with four discrete high speed diodes. This alone made a noticeable improvement
in the sound.
Your higher priced pre/pro should have much better quality parts, a heftier power supply and higher quality circuit boards.
It will also have more types of digital processing.
I will be getting an Outlaw 997 when it comes out. While it is not a super high end pre/pro it is sold direct so it should be equal to a $3000 US unit. Look it up and notice all the features. If Outlaw didn't have to pay for the licenses for all the processing it
would cost a lot less. http://www.outlawaudio.com/products/997.html