Diki: yes the MS actually sends AT messages, what I haven't been able to do is to change the effect in the AT message itself, for example as a mod wheel, resonance etc for the EXTERNAL keyboard. The MS sounds do respond to those parameters changes. My Korg TR as far as I know does not offer the possibility to send specific AT messages. I think controller keyboards send a "pure" AT message that modifies the slave sound according to how it was programmed OR a specially programmed AT message as Mod wheel etc. My MS is used in a very primitive way since I have no interest in wasting my time learning how to program a computer (at the moment). The worst thing of it as a controller with this old OS is that I haven't been able to adjust the sensitivity of the keyboard (with the very precise set of parameters it has) for the EXTERNAL keyboard NOT for the MS sounds, so I have to play more softly to get the desired effect with the SA voices.
On the other hand the PSR senses AT messages but it doesn't modify the sound because there is no editing possibilities for AT in the sounds. The PA500 has editing parameters for AT and the internal sounds have different responses to AT according to how they were programmed. I just wondered why the PSR has a special item for AT in the Midi set up if it is useless. If you program AT as a mod wheel in the controller keyboard the PSR will receive a mod wheel message not AT message I guess.

Hi Abacus: yes I already checked the AT receive item, one of the few filter options the PSR has, was activated.