Silly, there is no "there"
You will eternally find fault with one or more of your components. Upgrade one, and it will now resolve better and reveal a fault in one of the others.
This is how otherwise rational people work their way up to car priced audio jewelry
That said, it's tough to tell what is going on from your description. It could just be that you have many hard surfaces in your room and this particular combination of speakers and amp are thus too bright for your listening area.
Could also be something weird going on with that Schiit USB to spdif adapter.
Or maybe your ripping/encoding is the problem?
Try listening on other devices and other hardware and comparing and seeing if you still hear it. This is probably the only way to get down to root cause.
One thing I would try is to (just for testing purposes) wire a line out from the macs 3.5mm audio jack straight to the RCA ins on the amp (bypassing the USB and Spdif setup, as well as the DAC in the DAC/Amp) and seeing if it still sounds the same. If it does, then you know that isn't the problem and can move on to something else.