Hi Steven - welcome to WBF
This is really an excellent question and one which harks back to what Steve asked much earlier on - what makes my DAC sound 'analog'? Firstly a little background - I've been working on DACs specifically for perhaps 3 years now. I started out with an S-D design but in optimizing that I noticed that the faster the opamp I used for I/V conversion, the better sound. By faster I mean more gain-bandwidth, higher slew rate. This prompted me to look at the nature of what was coming out of the DAC chip - I found very fast edges with rise times of the order of 1nS (its a guesstimate as my scope isn't that fast). Reflecting on this it wasn't surprising as the DAC itself is a small geometry CMOS part - CMOS digital logic has similar characteristics if you buy the 'advanced' stuff. This discovery got me thinking - how to 'tame' such high speeds?
One thing I found with the sound quality, once I'd installed a very fast opamp (LM6172) in the I/V stage was that the size of the feedback capacitor made a difference to the sound - smaller was definitely better. If you look at (say) TI's datasheet applications for their PCM179X DACs you'll see they use something around 2n2. I found something less than 10pF sounded best with the LM6172. But then the LM6172 can cope with such a small cap because its slew rate is in the thousands of V/uS. When I tried with such a small cap on the NE5532 it gave really odd sound effects, like birdies on an FM tuner. It could be fine with music playing but in gaps between tracks the birdies would start up.
To explore what might be the cause of this, I fired up LTSpice and tried simulating. I couldn't provoke any untoward behaviour, no matter how hard I tried (and I tried for days). So whatever this effect was, it certainly wasn't reflected in the normal opamp models. I gave up with modelling and decided the only way I could make progress was to carry on with listening.
Another 'data point' I came across was Lynn Olson's report of measuring the output of his PCM63 into passive I/V with a spectrum analyser. That's a piece of kit I don't yet have so I was very interested in what he found. Which was that the spectrum even from this relatively slow chip extended to 50MHz. This finding correlated nicely with what came out of my (much faster) S-D DAC chip.
The conclusion I drew from all this was that standard opamp circuits really could not cut the mustard at the output of DAC chips. When I think about what opamp I'd be prepared to use for audio, opamps with GBW < 10MHz I tend to think of as a bit suspect. So that's a ratio of audio bandwidth to chip bandwidth of 1:500. Applying this rule of thumb to Lynn's DAC output spectrum, I'd need an opamp with GBW of 25GHz. Not really going to happen. I needed to tame the edges before sending the DAC's signal into any amplifying stage.
(I'll carry on with this saga in a subsequent post and of course get on to your other questions).