Heya,
Sometimes I use my receiver as well (Onkyo) when watching a movie. It's hooked up via optical out from a PC to the receiver. When I plug in, I can definitely tell when I'm on the receiver vs using a dedicated DAC/AMP. The thing is, the receiver doesn't have nearly as nice a DAC as something you'd get stand alone (at least in most cases unless it's a high end receiver). Between a soundcard and the quality of the receiver, I'd imagine if you heard your audio on a quality DAC with even no amplification (unless absolutely required to drive) you would be impressed. I can usually hear a weird noise in the background during silent moments. Highs sound different. Bass is more muddy than controlled. At least to me.
I would not get a USB DAC just to plug into that receiver. The receiver isn't just amping the signal and sending it back out, it's likely processing it. I could be wrong there, but I've found whether I input from my DAC or just optical in from the PC, it sounds the same from the receiver on headphones--which to me says that the receiver is processing the signal itself with it's own built-in DAC and degrading quality compared to my actual DAC.
I think if you tried something like a Hifiman EF2A you might be surprised compared to even a nice mid-range receiver. And if you tried a Matrix Cube DAC, you'd be wanting new headphones (not worth it though based on your headphones).
Very best,
