Yes, there's no accounting for our excellent taste!
And I came very close to ordering a Sonnet 2, but decided to try tackling my HD800 with a good NOS DAC first, so my chain will still be more sterile than yours:
FiiO X5 Coaxial Out > Octave MKII > OPPO HA-1 amp, balanced out > unmodified HD800
---
Of interest to our discussion, above: I received an email reply from Cees Ruijtenberg, having asked if there are any audible benefits to be enjoyed by using a PC to upsample from 44.1 kHz to 96 kHz in advance of playing a file through the Octave MkII.
I don't want to quote him directly without asking his permission, but paraphrasing, it boils down to this (my summary):
1) Depending on the software used, upsampling can create pre- and post-ringing.
2) Artifacts can be created that give the impression of more detail, but it's not genuine.
3) It's best to use the original files.
4) If you want to experiment, however, 44.1 should be upsampled to 88.2 or 176.4, not to 96 or 192.
In my original query, I hadn't mentioned anything about doing this to compensate the 3 dB at 20kHz attenuation that I've read some NOS DACs suffer. I had only asked if there is any audible benefit to upsampling with something like dbPoweramp - for the Octave MkII. Apparently, Cees feels it's best not to do so.
By the way, Cees responded very quickly to my email. It's great to have direct access to the designer. I'm humbled and impressed.
Mike