Thank you for answering that for me. I thought maybe we were all crazy to pay for the same thing at 2-3X or more of the actual price.
The Grant Fidelity in-home trial is great, for just the reason you mentioned.
Opamps - it's a never ending debate. Some people claim to "hear" each opamp, going so far as to say "opamps X is darker than opamp Y" and things like that. I find that to be overly simplistic. What you have is a complete circuit, usually rather complex, with the opamp being just one component of many. In some situations, opamp X may indeed produce a darker sound than opamp Y... but that doesn't mean X will always sound that way in every application. People seem to think of opamps like a CPU or video card - plug it in to any motherboard and it will always perform similarly, and always compare similarly in relation to another CPU/GPU. That just isn't the case.
It's like when people talk about "discrete" being the best possible thing that could ever happen to an amp or a DAC. Just like opamps, a discrete design could be amazing or could be garbage, depending on how well it is done. When you have a great designer like Yulong who has all sorts of tools at his disposal (which I'll discuss in my review), and who has a history of using both opamps and transistors in prior designs: I have to assume that when he created the D18 he used the best parts for the job.
It depends on your philosophy and what you mean by "upgrade". You could probably find some parts to swap out for boutique stuff. That would certainly be considered an "upgrade" on some levels.
Would it actually sound better though? There are people around here who would say yes, absolutely it would. Even without hearing it they just know it would. There are others who would say no, that boutique parts are a waste of money compared to reasonably high quality alternatives (which are already being used in the D18). You have to decide for yourself which category you would fall in to.
For me, I again put faith in Yulong. If there was a simple way of upgrading the D18 with some fancy parts, and it would cost maybe $100 or $150, wouldn't it have been built that way in the first place? But maybe they had a target price to reach and had to limit parts choices to hit that goal. Who knows?
Based on several designers I've spoken to, the demo kit has a disappointing power supply. It is decent, but not up to the task of extracting the last bits of performance from the chip. The analog components and system clock are also areas with potential for improvement.
The D18 may be a good match for you and your HD800s. It kind of depends on what amp you will end up using. The Anedio paired with a super-revealing potentially bright amp like a Gilmore GS-1 will probably be too much; D18 would be a better match there. But the Anedio plus tubes combo is a great way to avoid harshness but get every bit of detail and texture tha can possibly be had. It really depends on what you are looking for out of your HD800.
Really, you can't go wrong with either of them. If you don't need USB, and won't use the built in headphone amp, and like a somewhat smooth sound on the top end, then the D18 will get you there and save money in the process.