You do have reason to be concerned. That doesn't sound right.
I suppose it might be a synergy problem with the Mac out (which I have never heard). Just a wild guess. I can say quite honestly that my ES5 with the Solo + Pico Slim is producing sparkling treble. Percussion (high hats, cymbals etc.) are super crisp and realsitic. Admittedly Mac vs. Solo + Pico Slim may not be a fair comparison.
Another possibility is that some people just do not like the sound from certain equipment. For example, I did not like the iBasso T3D sound signature, and I pretty much hated the Shadow, yet both these amps have their fans. Maybe the ES5 is not for you. Keep at it though.
I agree with you that it shouldn't take 300 hours to burn in the stock cable. Actually I don't think that it requires burn in at all. I can't comment on the Twag.