This is something of a specialised & undocumented topic so will be happy to take it over to email if necessary.  Hopefully this is the right forum as it is Totalmix related.

The dev team may remember that a few years ago a tweak was made in the RME implementation of the kAsioSetInputMonitor mode of ASIOFuture() to extend the control options.  This is still working very nicely.

I have only ever used this on the HDSP 9652 and HDSP MADI cards, but I think this is part of the baseline driver so should work on any of the products that support the 'Totalmix' functionality?  Are there any exceptions?

I am assuming that all I need to do to change between any of the different hardware types is to change the channel count/index to match and that otherwise the command is identical, and as such I can use any RME product with the same ASIO code. 

The other question is what happens for the case where multiple identical cards (eg. 3xMADI) are installed. 

Obviously it isn't possible to mix between cards, but are the cards addressed as a single unit?  Eg. open the RME MADI device via ASIO and it appears as a (for example) 192 channel card?  In this case I have previously assumed that the channel index is one big space and that when using kAsioSetInputMonitor I could send indexes grouped in the ranges 0->63, 64->127 and 127->191 to control the mixing on each card.  Cross mixing between cards wouldn't be attempted as inputs and outputs would only be controlled in the same range (eg. mic inputs 0->63 could only mix to outputs 0-63).

Hopefully the above makes some sort of sense and this isn't too difficult to answer.