How are you hearing no difference?

Because you are not listening blind. If you did demo blind, you’ll find a $500 DAC and a $2500 DAC (assuming cream of the crop for their price points) are not going to sound dissimilar.  
  
If an amp or preamp has good linearity, low distortion (THD+IMD), high SNR, high channel separation, etc., then it’s hard for them to sound appreciably different. 

I don’t think listening blind will do any good. The problem appears to be people don’t know what they’re listening to or how to analyze sound in comparative situations. Obviously any tests are system and listener dependent so negative results should be taken with a grain of salt.

mzkmxcv

If you did demo blind, you’ll find a $500 DAC and a $2500 DAC (assuming cream of the crop for their price points) are not going to sound dissimilar. 

That's quite possible. Many poorly-conducted blind listening tests obscure audible differences between the devices under test. Conducting a scientifically valid, double-blind listening test is trickier than it might seem.

Not hard to believe. People hear differently. I grade all my albums on sound quality, (class A, B, C, D, and F). When I have others over to listen there is almost always disagreement, with incredulous looks, over differences in opinion. YMMV...

Tom