Also wanted to ask for some clarification around which expansion boards are available and compatible with the HDSPe Madi and HDSPe RayDat.

Kind regards

Thanks for you detailed reply Ramses! I've enjoyed your posts a lot around here and the work you're doing to document some fine specifications of the RME gear is to be commended.

My budget is not fixed. My plan is to make future purchasing decisions scalable so I don't necessarily have to buy all of it at once - for example even without ADI-2 Pro I will still have a pair of converters available for mixdown.

I already own one Cranborne Audio 500Adat with 6 of 8 slots full. This particular product is a bit special since it offers much more than simply a 500 chassis. It also offers excellent 8 channel AD/DA conversion via Adat with converter RTL of 0.4ms at 44.1 and balanced XLR IO, 2 very powerful headphone amps and a very high quality 8 channel analog summing mixer. It's for these other features that I am keen to get a second one. Apart from building my collection of preamps, compressors and EQ's a second unit will give me true 16 channel analog summing for mixdown back to digital via the ADI-2 Pro.

Regarding the UFX+, all the points you make I fully agree with and I would buy one straight up but for one shortcoming. It only has 2 pairs of Adat ports which means I could not get 16 Adat IO channels at 2x sample rates. Having to go to a MADI bridge just to get 2 extra pairs of Adat IO seems a punishment. The 12 analog IO's of UFX+ is a big plus too and to be honest a RayDat and 16 500 slots would not give me enough utility IO for things like headphones and live synth inputs. The other competing interface in this area is the Metric Halo LIO-8 3d and it has the ability to fit a 4 x Adat pairs add on card in addition to it's standard IO - but I have not been able to reliably confirm what it's RTL performance is. It too has some future proof interfacing options.

Currently and for 10 years my interface has been original UFX, UC before that and Multiface before that. Even with existing UFX I can add a second 500 chassis and ADI-2 Pro as discussed. This alone wold give me a significant fidelity upgrade and my RTL via these Adat converters improves by 0.5ms to about 3.5ms at 32 buffer. The UFX+ I know would improve that by around 1ms - so great. My target is to get2.5ms rtl at 32 buffer or better.

My current computer is a 2012 Mac Pro with 12 core UAD system. Other than the most simple projects I never work at 2x sample rates now but with an upgrade to current model Mac Pro and all that extra power I feel this is something I will want to explore.

So it's for all these background reasons that I think the PCIe solution may be best because then I am not paying for anything I don't actually need. Yesterday I said I was leaning towards the RayDat but then I realised I might not have enough analog IO for things like headphone mixes on bigger band sessions. Today I feel maybe the HDSPe Madi + Ferrofish PulseMX - but this system leaves me with one problem - nowhere to connect up an ADI-2 Pro - thus my question is it possible to have more than one HDSPe card in the same computer and have totalmix see both like one big interface? I've read somewhere this is possible but I would love some confirmation on this.

Kind regards

Yes 3 hdspe cards possible. In 1 totalmix session but in 3 tabs. Your daw will see it as one interface, but you can not route audio latency free from one card to the other in totalmix without a connection (you can use adat or spdif for that) . You can route from and to all cards and outputs from your daw (with latency set by the buffer).
Hope this helps
Vincent

Working with two TM FX instances is possible.
I personally regard it as limitation, not being able to route channels between two recording interfaces.

Additionally it might be nice to have the optimizing driver of the HDSPe MADI FX.

Having two MADI busses can be an advantage, because then you can distribute devices according to the way how you remote control them. If I recall right, then i.e. AUXDEVICE (shall you get an XTC or 12Mic) and MIDI Remote can not run on the same MADI bus. So you could distribute the devices then between the two MADI busses according to that remote management you choose.

Leave out the daughter card and you will have two optical MADI busses.
But this daughter card has WC, shall you need it in the future.
I don't have the time now, but I think in your setup you will distribute clock by MADI and AES to the ADI-2 Pro.

If you do not want to see all channels, then you can use the TM FX feature in "channel layout", to hide the not used channels.

Pls cross check with RME to validate the setup with the RME ADI 648 and Cranborne Audio 500.