Thanks for testing MIR 3D, @Mavros!
@Mavros said:
As the instrument dry signals are all in stereo this is the only way I understand to get them into MIR.
Exactly. Following the idea of working on the level of individual instruments MIR always expects stereo- or mono-sources, with stereo being the preferred input format. There is no dedicated 7.1.4 input (although I've seen people building themselves a setup with mono sources used as "virtual loudspeakers" in a surround- or 3D-setup 😉 ...).
@Mavros said:
This works well but apart from the instruments icons I also have the icon of the FX channel itself labeled MIR (the track name). What does this icon do and where should I place it in the venue space? It seems to be sort of a group channel. When I mute it. it mutes all instruments routed to the FX but does it also generate reverb reflections and therefore has to be panned and dosed correcty?
This question would be easier to answer when looking at a few screenshots, just to be sure that I see what you're seeing. I suspect that you simply talk about the (somewhat strange) way Cubase presents MIR 3D used as panning device in a mixer channel, but maybe I misunderstand your issue.
@Mavros said:
Another small issue with MIR on a 7.1.4 track you always get a warning that the ambisonics setup which is designated as 7.(1).4 only 11 sound sources instead of 12. I guess I can just ignore that as I do not route anything to the LFE. my subwoofer's bass management is done by the audio-interface.
Hmmm ... as long as you don't actively select a "raw" (i.e. un-decoded) Ambisonics setting as your Output Format, you shouldn't have to deal with Ambisonics channel counts at all. As long as the channel count of MIR 3D (as a plug-in) is identical with the channel count of the chosen Output Format, you shouldn't be seeing a warning message. A format designated as 7.(1).4 in MIR 3D will take the LFE into account, but it won't use it for any kind or output. Formats designated as 7.0.4 will ignore the LFE also for routing. - This is always noted in the short description texts I supplied for every Factory Output Format:

BTW: You must not mistake the LFE as bass management. The LFE is a channel for dedicated low-frequency content that is neither part of the signals presented in any other channel, nor should it correlate with them (read: it shouldn't carry tonal information). That's why MIR actually doesn't offer many (if any) factory presets that feed the LFE.
@Mavros said:
A last question. I use the instrument role presets which apparently also set a volume level. example the timpani is set to 3 dB and a flute to -18.5. I think this is the parameter natural volume. My question however how does the volume setting in the source plugin come in into this setting?
For SY library signals there are two volume parameters, CC7 and the actual microphone level. For now I set all close signals in the plugin to 0 dB which is of course relative as CC7 for the timpani is set to 90 whereas for the flute it is set to 101.
"Natural Volume" is a feature that was designed exclusively for the use with Vienna Instruments, now also known as Vienna Studio Series. I strongly suggest to disable it altogether in the Preferences when using MIR with any other kind of audio source, including Synchron Series.

When mixing with MIR, it is also advisable to decide which of the many volume control options in a modern DAW are used for what. Personally, I suggest using MIDI volume only for performance-related tasks within a single instrument or ensemble, while MIR volume is used for (static) Venue-specific changes to the overall balance of the active instruments. Finally, the actual automated fader rides for mixing are performed in the DAW.
HTH!