Quite a few Bricasti M21 DACs for sale cheap

I think even the M3 has a one bit DSD converter which is selected by switching to NDSD option. That’s native DSD. You can toggle between it and the PCM. Or may be it’s implemented differently in M21. 

I hear good things about using either Roon or HQPlayer to convert all music to DSD, then feed it through the one bit converter on the Bricasti DACs. I find that almost always sounds great even when using regular Delta Sigma designs, so should work excellent with the one bit as well. 

Actually Bricasti doesn’t recommend upsampling before the DAC. I believe it’s in the manual. They’re of the opinion that the DAC sounds best when fed the original bitrate. I tried upsampling to DSD using Lumin U1 Mini and the built in network card as a roon end point using roon dsp. In both cases I preferred the sound when feeding the M3 the original bitrate. 

I also tried upsampling to DSD using Audiovana on my MAC computer to my Bricasti M21 DAC.

After listening and comparing for a couple of days, I also preferred the sound when feeding my Bricasti M21 DAC the original bitrate. I usually listen using the ladder DAC but sometimes switch to the delta sigma DAC.

Regarding the OP post, “Quite a few Bricasti M21 DACs for sale cheap”, many components are always for sale. I suggest you not worry about it and purchase what sounds good to you.

Based on my research, it does not appear that Bricasti recommends upsampling before the DAC.  The Bricasti M21 DAC provides multiple processing options, including sigma-delta, R2R, DSD as DOP, and DSD in analog. This suggests the M21 can handle different input formats without requiring upsampling. Please call Bricasti and ask them.