It is well known that no processor currently available can do DSD 512 with the EC modulators in HQP. But, no need to fear, because the AKM DAC chip does better with DSD 256 than it does with DSD 512. This is right in the data sheet for the AKM 4499. So best performance with this DAC chip is at DSD 256, not 512. This is likely because the switching elements in the chip are not fast enough for the higher rate of 512. In fact, Andreas Koch has mentioned a few times in interviews that he believes DSD rates above 256 are a bad idea, because no switching devices are fast enough to remain fully linear at this rate.
I no longer have the D-90 here (sold it), but I preferred it with DSD 256 inout from Roon (linear phase filter, precise, 7th order CLANS modulator) with the D-90 in "direct DSD" mode (aka "DAC" mode in the set up menu).
BTW, the poster above is not correct when saying D-90 does not run in a non-oversampling mode, in DSD direct there is no oversampling. But there is not a non-oversampling mode for PCM input.
Do you know how NativeDSD can upsample to DSD512 with EC modulators? What if instead of sending the stream to D90 you can output to a DSF file and play that file back. That would use 0.1% CPU utilization instead because the computer is NOT filtering realtime and just sending the stream to the D90 at 45.2 Mbps. Any low powered PC can also play DSD512 with no hiccups whatsoever if played from a file rather than realtime oversampling/filtering
Side note: Funny that those mastering studios use a terribly measuring Nagra HD DAC X as their reference DAC lol