These are not separate components like Preamp in a case, DAC in another case, headphone amp (if applicable) in another case, etc.
Just take a look at the interior photo of the product. So I can't answer question like "just compare the preamp only". It just doesn't work like that.
When the engineer design a $1795 product versus a $799 product, he is already given certain constrain about the use of parts and cost.
Sure, the preamp on DAC-10 and DAC-9 are using the same FPGA chip, but there are other factors in play here, such as power supply, power circuitry. Having said that, the cost of DAC-10 design is actually for certain features such as 2 level of gain settings, circuitry that allow for high power headphone amp (DAC-10H). So those of you who like warmer sound (typically you're not listening to classical music), would consider DAC-9 and DAC-10 at the same level, and prefer DAC-9 over DAC-10. But if you have a power hungry headphones (or a high end one), you would save a lot of money with DAC-10H (a top of the line headphone amp alone could cost $1000 or more). DAC-10H was compared to a $3000 headphone amp by a reviewer (he sort of gloss over in the review but private told me in more details) and he can't tell the difference.
I probably go off topic a little and my purpose is to educate customers about various design trade offs.