DAC Hat vs. Other DAC?

[Taki8888]

Hi,

I have the IQ Audio DAC hat/rpi4 set up. 

I don’t have the deep knowledge in regards to the technical side of this.
I read some posts mention USB is dirty noise wise and affects sound quality.
In what application would this apply? PC to DAC?

For the rpi and DAC hat, communication via I2S. If one plays music from
USB flash drive or USB SSD, does this USB noise still apply?

If you have experienced the DAC hats vs. standard DAC like Topping D10 or
SMSL SU-8, SU-9, is the Sound Quality night and day differences? Or subtle
improvements?

Right now I’m considering the Allo Boss 2 but not sure what I would gain
from it vs. the IQ Audio or trying the SMSL DAC which is 4x price of the DAC hat.

Thanks