Overall I think that too much attention is being paid to the supposed commonality between the M and the DX90. I have a Stoner Audio DAC using the full Sabre chip and it is nothing special (nor at $50 do I have much complaint). It is all in the implementation - especially with the Sabre chip. The second point is the amp sections are different and for this we can assume no commonality whatsoever. (I actually think that the Rockboxed DX50 is a nice source through the line out. However the internal amp is kind of lifeless. The DX90 is meant to improve this; here is hoping.)
My point is that the M will rightly be compared to all of the competitors out there - not specifically the DX90. Its price will be justified or otherwise on the sum of what it brings to the table. I personally have high hopes but I want to hear it before I commit. I am especially curious to find out whether it can do what I require without a further external amp.