How are you hearing no difference?

You have to ask yourself, what makes them different.  For example obvious SS vs tube sound.  Then what are the caps and resistors used.  Power supplies.  I have mentioned this many times, the Counterpoint line is an example, the basic preamps use 1 6DJ8 tube, the next model up 1 6DJ8 per channel, then as you go up the line separate power supplies, tube regulation and tube rectification, so you get the picture.  If you are comparing similar designed components then they may not sound exactly alike but maybe not that different also.  Again tube vs. SS sounds, Class A and D designs, OTL and DHT designs, see where this is going?  If you only compare preamps that use 6922/6DJ8 tubes with similar designs, they may sound different but maybe not as much of a change that one may be looking for.  Probably better trying a 6SN7 preamp to see what that would sound like in ones system, etc.

Then there are people who can change the sound of a component with modifications, that can be as simple as better power supply caps, swapping out resistors with Vishay, caddock, shinkoh, swapping out other capacitors with V-Caps, Jupiters, Duelands.  And then there is adding filter chokes, upgrading transformers, etc.  These simple changes may change the sound to more what you are looking for, plus once you hear what these changes accomplish, this can then help you understand each design and what changes make a component sound the way it does in comparison..

Happy Listening. 

+1 to tomcarr.  Everyone hears differently, listens for different things or perhaps not as critically. 

Because people like to think that they are smarter than everyone else and that "they know better." They’re just trolls who have "be right."

It's also possible that their system are set up like some of the ones that I've seen posted on the B&W forum on FB where one speaker is sitting in a corner, they aren't toed in equally, there's a lot of exposed glass in the room...  If the whole system sounds like garbage anyway, then they won't hear a difference between cables or a DAC.

I really like the way my system sounds. I often wonder if it would sound like garbage to other listeners.

Like garbage probably not, like not much to keep talking about probably yes.