For such a high end unit, there appears to be very little information about which formats the McIntosh MX-121 does support.
Their web site says:
Dolby Processing True HD/Digital Plus&EX/Pro Logic IIz, IIx
DTS Processing HD Master & High Res. Audio/ES/96/24, Discrete & Matrix6.1/Neo:6/Express/Neural
I have to assume that the 96/24 refers to 24bit/96kHz audio, and that decoding of DTS-HD MA is therefore restricted to 24bit/96kHz. If that is the case, lacking any other information I can find anywhere, I would assume that Dolby TrueHD is also restricted to 24bit/96kHz. Of course both
DTS-HD MA and
Dolby TrueHD are both restricted to 24bit/96kHz for 7.1 surround sound, and as the MX-121 is a 7.1 channel system maybe it has been restricted to 24bit/96kHz when decoding those formats.
Page 40 of the User Manual gives a hint of capabilities as well, in the "Compatible Formats" table for USB drive playback, it lists FLAC format support up to only 96kHz. See attachment. This implies that the MX-121 can only decode FLAC up to 96kHz.
I found some limited evidence that
it can support playback of 24bit/192kHz audio from Blu-ray Discs, but there was no consistent information or thorough testing that I could find.
This article lists the DACs used in several McIntosh units, but not the MX-121. However McIntosh did send the Datasheet for a 24bit/192kHz chip, so it probably does have a 24bit/192kHz DAC.
Maybe it can play stereo PCM at 24bit/192kHz?
When you up-convert music to 24bit/192kHz, are you outputting it encoded as DTS-HD MA or Dolby TrueHD, or sending it as uncompressed PCM?
If you are encoding the audio to DTS-HD MA or Dolby TrueHD in MC DSP (is this possible?), can you send it as stereo PCM instead? ( I don't have the equipment to test if MC would accept this setting.)
If this won't work over HDMI, the MX-121 has optical inputs that do accept PCM. Optical S/PDIF connections can support stereo only 24bit/192kHz uncompressed PCM, so if your computer has an Optical out connection that supports it, try connecting your computer to the MX-121 using a TOSLink Optical cable, and see if that works. If it does, you could set up a zone that switches output of your computer from HDMI to S/PDIF when playing stereo 24bit/192kHz audio, and set your MX-121 to that input.
Anyway, those are some things to try. You could also try asking McIntosh again directly which inputs support 24bit/192kHz and what formats are supported. i.e. DTS-HD MA or Dolby TrueHD, stereo or up to 5.1 channel.