Besides some of the ARM options like the Honeycomb that have been mentioned, there are also the POWER9 systems from Raptor Computing Systems. Much more expensiv

Besides some of the ARM options like the Honeycomb that have been mentioned, there are also the POWER9 systems from Raptor Computing Systems. Much more expensive than the x86 systems, but I feel compelled to mention them, since they have mATX boards with most of those yummy features (fewer SATA ports but higher RAM capacity, for instance).