Yeah it does look like a tough sell against the slightly pricier Lyngdorf MP40. Alot of it will come down to the quality of the DAC implementation, they are speccing 5.0V output which is fine but 0.005% THD isnt good.
The MP40 offers Room Perfect and Auro upmixer for not alot more.
I just returned an Arcam AV40 which I had on trial and the dealer said he was pretty much fed up with the problems with Arcam units (when I was waiting to be served I saw 2 boxes being sent back to Arcam, one was clearly the same processor).
He said he was looking for another processor to stock in the same price range and mentioned the new McIntosh which must mean this one, suggesting I test it out when it comes. I may well do, but if I'm jumping from £3kish to £6.5kish (he mentioned £5k but perhaps that's the dealer price) then I'd start considering the Lyngdorf too!
4 hdmi in and 1 out really is an odd choice.... I use 5 in myself and 1 out.
//edit looking at a picture of the MP40 I see it has 3 hdmi in, having read the review of the MP60 it didn't occur to me there could such a difference between them with regards to input.
So the MX123 gets a purely digital baby brother. Half the height, half the price, half the HDMI inputs, 11.2 instead of 13.2. The lack of analogue inputs makes it a no go for me, but for somebody who just wants a high end HT processor, it could be interesting. Pity that RoomPerfect is only available on the MX170.