Hello Nick,
I've been using an M-Audio Keystation 61 Mk 3 with my keyboards for some time now. As with most things there are good and not so good points.
Connectivity is easy, just a midi cable and power (printer) cable. I use a mobile phone 5v charger for power as the board draws less than 50ma.
Key feel is semi-weighted, piano type and not that good, especially as the keys are wider than the SX900 and so do not line up over entire length.
One good point is the distance between the back of keys and rear face of case is only about 25mm, so the two fit reasonably well together.
The only controls I use are transpose and octave shift so cannot comment on the rest.
Be aware the board has to be set up to produce a midi out signal and there is, or was, an error in the manual procedure.
Would I buy again? Yes, probably, it does all I want for a simple setup for a hobby player.
Regards,
Robert.