Get a Roland, JV 2080 (that's waht I use) or something a little better from the xx80 line. These models are expandable too, with sound module cards you insert inside the box. That way, you can customize your sound module for the style of music you want to produce.
depends what sounds you are after ed. but the top range Roland or Kurtzweil are the way to go.....or......get giga studio, it has some great sounds especially it's piano.
Yeah, I have a Fatar SL-990 too. And a Fatar Studio 90.
I love them both. But he was asking about a sound module, not a keyboard.
What do you use to drive your 990?