I owned an early 80's Sigma dr-35. It was stolen however, I have been on the lookout for any older Sigma dreadnaught, models Dr-18, Dr-28, DR-28H (herringbone bindings), another DR-35 or the DR-41 ever since. These were all solid spruce top replica's of the Martin dreadnaught line and the top axes of the Sigma line as well. The main difference I could see between the two (martin and sigma)was the composition of the finish. The sigma finish seemed to be heaver and tougher and not as apt to check in the cold and considerably more durable at the price of less resonance with the top. Other than that, I think they were identical to the martin counterpart except in price and re-saleability. As of this post, I cannot difinitvely say back and sides are solid or ply however, happy days are here again.
I finally found a DR-18 that some guy sat upon and broke the neck block or at least somewhere near that area. The back also started to "pop" out of it's binding at the area where the neck meets the body. Ma The guitar is in rough shape and is going to require extensive repairs, however I'm hoping it was not put together with exoxy or something that won't soften up with heat. It is also the perfect excuse to grind off all that heavy finish that made most Sigma's bulletproof and try to allow the top to resonate like it's american counterpart.
I have also had the experience of playing some of Sigma's lesser line such as the DM-3 and found the plywood top Sigma's were pretty much a beginner's guitar and not really suited to more experienced players and no amount of re-work will ever make them better.

Anyone having any experience in repairing Sigma's I would be most interested in hearing your findings on how easy they come apart.
Thanks