Can I connect directly to a receiver's DAC ?

In answer to your second question; "... how are the DAC's usually, in upscale receivers from Yamaha or Denon compare to stand alone DACs..." - Based on my experience, these internal DACs are only comparable to the sub $400 stand alone DACs, and even then, there are some fairly amazing "entry level" stand alone DAC that sonically crush the "internal" DACs from inside a receiver. As for more expensive stand alone DACs (above the $1000 price point all the way to the moon) many can be in another sonic league entirely. As you "climb the digital ladder" many will have a much larger soundstage, a much better front to back depth (which is the absolute "bane" of most cheap digital IMHO), a much more realistic high frequency portrayal (less grain, much better high frequency overtones) and offer a better ease of the overall sonic presentation - much closer to analog.