Could Class D really be that good?

Yes, the amount of distortion and its spectrum has some correlation to sound. However, it is not the major or only thing that makes and amp SOUND the way it does. You can give me 3 pairs of Atmasphere amps and I will change some of the parts in one to really bad sounding parts.....leave one of them stock and in the 3rd one I will change a bunch of parts and execution that I know will make the sound better. They will still all measure the same. Ralph or anyone would be able to pick them apart..........even on a double blind test.

When you change parts, you change the distortion so the sound changes. This is easily measured and correlated to what we hear. So they would not measure the same. Distortion is literally the sonic signature of any amplifier.

Did you not read what I said?  I said the amps will all measure the same.....this is absolutely true.  While it is true that when changing parts the distortion changes....because all noise and veils are distortion.  However, you CANNOT measure a change in measured distortion when changing passive parts.  I can hear the difference in 2 inches of wire.....try measuring that!  We have been down this road many times.  You are one of the few people on the planet who BELIEVE what you just said.  Again, you cannot measure the difference in distortion by changing passive parts, damping, fuses, cables, etc.....no way Jose!  Lift your cables off the floor.....you get lower distortion....try measuring that!  Try removing the steel plate and bolt off your toroidal transformer and raise it off the chassis with a one inch piece of wood and be amazed at the sonic difference....try measuring that.  Please, come over to the "dark side".....it is way more fun than re-stating the same BS.  You will learn a lot and your products will get even better and you will make more money.  A win for everyone.  But many will take their "opinion" to the grave.....It is called being "dead right".  I am wrong many times.....I change my mind....it is fun....it is an ever changing game....infinite and full of surprises.....I learn new sonic things most every week.

I definitely have no experience with op amps but believe in their ability to change a piece of equipments performance. After building the Nilai the first things that come to mind are wanting to upgrade the jacks along with internal dampening. The caps could be a major hassle due to space..the wiring not so much.

As much as I love the beautiful inaccuracies or pleasant mild distortions/coloring of tubes I do appreciate low noise linear solid state gear as well. I pulled a tube pre off the Nilai yesterday and enjoying now straight from dac. With certain music I prefer the visual perceptions I can get from tubes and a clean power amp. Kinda funny how I strive to find & appreciate an amp like the Nilai due to its engineering and impressive specs & then look for different ways to add changes to its sound. Not with just passive parts. 

Whilst Discussing OP Amps, the OPA 1656 is one known to be selected for use in place of other OP Amp's.    

I first heard about the 1656 from a description of the LSA Discovery Warp 1 amp. I bought some and put them in place of the 1612s in the Purifi factory input board. Did not like them, not as musical as 1612. You might love them....but trust only your OWN ears.....not what others say. Most people act like sheep.....just following and repeating what others say somewhere. How about the "You just need to have the impedance 10 times higher for a preamp to amp match".....another words, a 100 ohm output impedance is fine driving an amp with 1K input impedance. Who the heck made up that statement? But you see it repeated on forums over and over. My 50 ohm output impedance source sounded better with a 150K load than it did with a 50K load. How about bypssing caps....."The rule is that you need a bypass cap that is one tenth the value". What a bunch of nonesense. You need the value and type that sounds best with the main cap. Many use .02uf to bypass large caps. You must listen to know. You must love to really know love. Please Love more......more often and more deeply. Makes you happy.