I´ve been doing research and I´ve found that nowadays PCI Slots in modern Motherboards are bridged and could represent a problem regarding stability and performance of the soundcard and system.

Having said that and reading all your post (which I really appreciate and I´ve learned a lot) I think I may go for the RyDAT card with two ADAT systems and mount them on a custom build rack.

I´ve also read about a brand called MARIAN , and they make a card called SERAPH. I´ve read that their performance is extremely good and they offer a version with 8 analog outputs. Anybody knows the brand? Any experience? 

So returning to the Mini PC idea , I could mount the PC with the ADATs too. Check this PC Mini ITX case: 
 

 
My only concern is about the cooling and the noise comming from the CPU fan. Could this affect the Soundcard? Do you think this idea could be a succesfull one?

That case is nice and small (portability)... but that's about it on the positive size.

• The CPU cooling would be terrible (undersized and loud).
• PS would be weak, possibly loud, and may be proprietary.
• Your PCIe audio interface would have to use a riser-card and mount horizontally. That's also a potential spot for trouble.

Ps would be weak? what do you mean?

PS= Power Supply
 
I agree complete with Jim. such a min itx is fine if you do not Need power and a small cpu is enough, such as home media streaming applications. and even there you have to carefully choose the components to be convenient with your application.
 
 

If you are going to go with a rack for the ADAT and plan to rack the PC, why not go with a full size server rack case that can accommodate a standard ATX board, your PCIe expansions, and a decent power supply and cooling?
 

i'm don't think the 9652 is compatible with a 64 bit os.

timidi
i'm don't think the 9652 is compatible with a 64 bit os.

The HDSP 9652  is, it is still sold by RME, the last driver (64 Bit) is from 9/2017
 
The Digi9636/52 Hammerfall is not compatible to 64 Bit- I guess you mean this one.
 
 

abacab
If you are going to go with a rack for the ADAT and plan to rack the PC, why not go with a full size server rack case that can accommodate a standard ATX board, your PCIe expansions, and a decent power supply and cooling?
 

This thought I had also.... it would protect the toslink connctions between card and Output-Devices also. 
But it is not really compact anymore..... 

SigiZ58

timidi
i'm don't think the 9652 is compatible with a 64 bit os.

The HDSP 9652  is, it is still sold by RME, the last driver (64 Bit) is from 9/2017
 
The Digi9636/52 Hammerfall is not compatible to 64 Bit- I guess you mean this one.
 
 

Yes, you are correct.

How about 18 X 18?  This is what I use:
http://www.music-group.com/Categories/Behringer/Mixers/Digital-Mixers/XR18/p/P0BI8
It is a digital mixer with incredible routing capabilities and also does 18 in x 18 out.  All on a single USB.
 
Just a thought.  Then getting Sonar to work would be the limiting factor.  Look at my profile.  I am currently using an i3 desktop (an OLD one).  Since I do not do a lot of MIDI, it is fine...If you do a lot of MIDI, you may need more CPU...YMMV.