Olders Ketron use FM technologie for FM sounds , MS série for exemple. I dont knows actualy on Audya or SD7 if it was same thing

Advanced FM that came with the SY series keyboards and now 8 operator FM on the Montage present some pretty impressive FM sounds way beyond the DX-7. But I still haven't played a ROM based keyboard that has the velocity controlled expression of even the DX-7 let alone more advanced FM releases. I am not saying FM synthesis can replace ROMplers but the combination of both is ideal. That is why the Montage is a very attractive keyboard.

Yes.. You are right... On my old SD1, I remember that...

This was the DX 7 Grand Piano:

http://www.synthmania.com/Yamaha%20DX7/Audio/INT/INT08%20PIANO%20%20%201.mp3