DAC Chips and Software/Programming

PS - If memory serves several DAC chips have a variety of components which can be bypassed in favor of external circuits, allowing the buyer to pick and choose which on-chip features they take advantage of.  The output filters and upsampling are two features I can think of off the top of my head.

Usually not the way you are thinking.  The DAC chips themselves are excruciatingly simple, but some amount of programming has to go into the design with any implementing upsampling.  You can't just flash them and change them to work one way or the other however, unlike PFGA based DACs like I think Chord and PS Audio offer today.  

I think it has a lot more to do with the implementation.    I have one DAC that is fully firmware upgradable,  has very good DAC chips and an amazing  host of DSP features .  Remote control,  can record to USB...Its amazing what it can do and it sounds very good

My other DAC uses 8 thirty something year old NOS Philips DAC chips with no oversampling and a discreet analog stage with tube output.  It has zero features other than Input selection and power button.  It sounds much better than the DAC with the latest and greatest chip set.  It's the implementation and high quality output stage that makes it sound great.