I’m not familiar with your receiver or with the DAT850.
As long as the SPDIF output on the receiver is configured to send PCM stereo (not Dolby or DTS), at 32 or 44.1 or 48 kHz, any DCC recorder can record it digitally. I assume this should be true for the DAT850 as well.
All DCC recorders can record through one or more analog inputs. If you make an analog recording, the recording is always encoded at 44.1 kHz. The built-in ADC in DCC recorders can’t sample at any other rate than 44.1, though some recorders (including your DCC951) can process 18 or even 20 bits per sample instead of just 16 bits.
Digital recording is only allowed from an original, but not from a copy. This is because of Serial Copying Management System, also known as SCMS. You can make a digital copy of a CD, but that recording cannot be copied any further unless you make the second generation an analog copy.
If you want to make a second-generation digital copy, you will need some way to defeat SCMS. There are devices you can buy to change the SCMS codes on an SPDIF line. I don’t have any recommendations; I use an SCMS defeater that I bought as a kit in the 1990s, based on an article in Elektor magazine. It not only resets the SCMS “copyrighted/non-copyrighted” and “original/copy” bits, but it also changes the source bits. This is necessary because some digital recorders (including the DCC951) have firmware bugs that misinterpret the bits in the input format. I own a CD-single where the audio was accidentally encoded with one of the bits set wrong, and my 730 refuses to record it without the SCMS defeater.
You can also work around the SCMS limitations by grabbing the audio with a digital audio receiver device on your computer, and then using a digital output (e.g. an HDMI to DVI+SPDIF splitter) to play it back.
Bottom line: you’re not doing anything wrong; it’s just the way it works. To make it work better, you have to probably buy some gear to work around SCMS. Or just make an analog recording.