The Orla "Digital Sound Expander" comes probably in different versions. I have a, so called, "DSE-12/24"; with room for an extra 12 voices. It is also possible to expand the 98 factorysounds with another set of 98 voices. But we are talking about the eighties. I don't think Orla delivers these expansions anymore. There's a big empty place on the mainboard where you are supposed to solder a bagfull of electronic parts in order to get the extra 12 voices. The Orla seems more to be a do-it-yourself-kit. The manual states that it uses "PM synthese". That would mean that soundgeneration is more close to the Casio CZ's than FM-synths. To my ears it sounds smoother than FM. Besides 4 operators and eight algorithms things differ from, let's say a humble DX100. Only one sinus-LFO for general modulation and no pitch-envelopes. Instead you get the option to double a patch with itself (making the Orla 6-voice) with a detune parameter. Also built-in on the mainboard you will find an "ensemble-chorus"unit. Not common on old FM synths. This 19-inch unit is more directed to the professional organplayer (Böhm, Wersi, Orla or Elka).
If you can find one very cheap you could buy yourself a friendly, ugly-built, "hybrid" of a FM/PM synthesizer, with some classic sounds. The 98 patches in RAM can all be overwritten, and if you go for the cheesy, eighties-sounds you can re-call these from ROM.