I'm going to answer some of my own questions here. I have an ADI Pro 2 FS BE that I use for room correction so I pulled it out of the stereo and did some basic measurements last Sunday.
Not definitive but it appears the SE300 does DSD->PCM conversion. The measurements for DSD and DXD are virtually identical (NOS) and there's no mention of a separate DSD pathway. There's also an FPGA available for this in the design.
(I like to use HQPlayer on devices with a dedicated DSD pathway to 'bypass' the DAC. Makes very little difference here.)
Measurements for both of the above are marginally cleaner than 44.1k NOS. At 44.1k distortion products on a 1k sine test produced the pronounced 'grass' you've likely seen on other R2R DAC tests.
I was able to get <= 0.006% THD+N in my measurements, as stated by A&K. I was not able to get the same SNR but I'm 100% that was user error on my part.
Class A measured a little better than AB.
I was not able to do any linearity measurements.
Take this all with a grain of salt. I'm not experienced at testing audio gear and I wasn't getting sensible SNR results. I was, however, able to measure some other gear I have with published measurements which suggested the distortion measurements I was getting were accurate.
To be clear, I do not consider this a flawed design because it doesn't measure as well as a $99 CS43131 dongle. I have a bunch of audio gear and the SE300 is my go-to most of the time. Measurements are part of the story.
