How are you hearing no difference?

I have had two DACs with the sound so close i could not believe it. Primarily because one was a used, at the time, fifteen year old DAC I bought for $250. The second a new highly praised on both TAS and Stereophile costing $2,300. I had them both when I bought a bunch of other new stuff. I returned the new DAC. Took eight more years for me to find a better DAC. Spent $7000 to buy a SACD Marantz SA-10 to use mainly as a DAC.                 
As for preamps I own six. and yes they all sound different.

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.