I see someone note that Siel was based in Acquaviva Picena, near San Benedetto del Tronto, Marche, Italy. I've heard rumor that the family in Italy responsible for several other musical instrument companies in Italy also started Siel. If anyone has information that would help let me know. As to the demise of Siel though check out this Sound on Sound article outlining Roland's decision to buy them out and close production. Also here's a letter from a fellow who used to work for Siel etc.
       Siel had a limited number of products of interest. Yet the ones they do have are certainly of interest in my opinion. The way they layed out the circuitry is significantly different from other manufacturers and this of course results in unique sound and features. Check out this great page for details and pictures of Siel Synths (Societa Industrie Elettroniche is where the name comes from by the way.). The author has done an excellent job and lists many units that aren't found other places on the web.
      While apparently there exist a limited number of VCO based units, the polysynths mostly use the SSM2031 as a high frequency oscillator. This feeds either a divide down network in the TMS3631 (as in DK600) or the M112B1 synth chip in the DK70, DK80, and perhaps others. Then waveforms are selectively filtered to form the sawtooth wave. This results in a warmth because the oscillators are free to drift from each other slightly when dual oscillators are used. Then they use the SSM2044 filter to allow for extreme resonance with a tonal clarity that makes the synths excellent for many types of sounds. And for the money you can usually get them for you can't really beat it! DK600 has only one adsr envelope that can be shared by vca and/or vcf. All selections are and/or in fact. And the newer synths have more elaborate enveloping but no knobs to control those features directly. DK70 is essentially half of a DK80 but has optional 'stage set' neck to turn it into a keytar. Enjoying mine finally after getting it repaired here!
      Construction is plastic (A fairly durable one it appears)on all the machines...the DK600 just looks particular retro..I call it 'milk carton technology' :-). But hey, it's all about the sound. And if you want to get a lot of it for a little, these machines may be top of the heap in analogue world. I heard another rumor today that possibly Dave Smith was in on the design of these machines. This would not be of surprise because SCI marketed their own overlay in the "Prelude" and the guts were an orchestra 2, and also the SCI Fugue was a redecorated Siel Cruise. Fugue came out in 1979 only a year after the Prophet 5.


Orchestra Mono Cruise Orchestra 2/OR-400 Opera 6/DK600/Expander/KIWI DK700 DK70 DK80/Expander 80

A Letter from (Name removed due to random people being pestered that happen to have the same name) (he has no parts for them!)

Hi Sound Doctorin',

more lines to give you other informations of SIEL (from your webpage http://sounddoctorin.com/synthtec/siel/siel.htm). 
I live in San Benedetto del Tronto (ITALY) and I have worked in BONTEMPI/FARFISA and for SIEL (you can see my curriculum vitae on LinkedIn).   
SIEL has born on 1976 by a group of employees of FARFISA (Camerano - ANCONA).   
SIEL have made many synths (CRUISE, Mono, DK, OPERA6...) and many electronic organs (the serious HB is the most famous).   
Their synths have also been sold with the mark SCI (Fogue & Prelude), ARP (Quartet) and KEYTEK (MDP40 Midi percussion, see http://matrixsynth.blogspot.com/2008/03/siel-mdp-40.html) also sold by SUZUKI named RPM-40 (I have an owner's manual of this very rare drum box).   
Before selling the building to the ROLAND ITALY, they have tried to commercialize other products with KEYTEK mark (example CTS200, see http://matrixsynth.blogspot.com/2008/01/keytek-cts-2000.html) 
You can find the CTS-2000 owner's manual in the following link: 
Other good product has been MK610, nice and economic keyboard.
you can find it at site http://matrixsynthb.blogspot.com/2007/08/siel-mk-610.html
Other interesting info at link http://www.teaser.fr/~amajorel/siel/
SIEL has definitely closed activity in 1988.
Now, in the building, there is ROLAND Italy.
(Name removed due to random people being pestered that happen to have the same name)