The main problem is the 'dumbing down' of MIDI capabilities in the 21st century arrangers, IMO.

I have seen a definite trend towards much less comprehensive external MIDI control over the last ten years or so. When MIDI was around in the first place, it was kind of EXPECTED that you would be hooking up different things to your arranger, and the ability to route individual Parts to either internal or external gear was more common. But as arrangers have gotten better, fewer and fewer players actually need to do it as much, so, as in all things, it's 'use it or lose it'.

If I remember rightly, I was informed that sending external Parts from an S900 was an 'all or nothing' affair, at least as far as the Style Parts go. Correct me if I'm wrong, but you can't send, for instance, the drum part in a style to a much better module without sending ALL the style parts to it. I am not sure about the Keyboard parts, but from what I remember, it's not exactly a walk in the park, either.

Especially if you want some registrations using the external gear, and some to NOT use it.

Roland are one of the better arrangers to allow Parts from the Song and Style section to address external gear on a registration (UPG) by registration basis, but even they forgot completely about the Keyboard Parts, which CAN address external gear on a UPG by UPG basis, but can't (without a convoluted workaround involving the sequencer) select ANY CC0/32/PC#, and can only send the codes that address it's own internal voices. So, if you need to send something that doesn't already exist, you are out of luck.

It's stuff like this that really gets up my nose...

If I were you, I'd download the manual for the S900, and take a look for yourself. But don't expect much flexibility, especially from such a mid-line arranger...