Not going to pick a 9018D up since I have my money invested on something else. Will need to start printing money to be able to afford all these toys. However, I do have the Oppo HA-2 with me, which also uses the ES9018K2M, and will be reviewing it soonish.
Got the HiFimeDIY USB DAC this morning and just run through some test. As expected, the basic topology is almost identical between it and UD100. The real difference is that HiFimeDIY sets the ES9023 to the lower max voltage output (about 1.84Vrms) where UD100 is set to full 2.06Vrms output. RMAA measure is also very close between the two. The only noticeable difference is that the lower output on HiFimeDIY seems to improve the overall stereo crosstalk on ES9023 a bit.
If you are keeping the budget under $100, then FiiO E11K might be your best option (provided you have a decent source with good output). Though if you push your budget a bit more, an FiiO E12 will be a better, and likely a safer choice.
What kind of setup / source were you using for your listening? Also, as far as I know, iDSD doesn't have an upsampling algorithm, so to speak, unless you are referring whatever software upsampling you were using at the test.