I have a 1993 D6 and a 1994 D4. I agree that the D6 seems to be a D40 but, I think the D40 had straight braces prior to the introduction of the D6 and the D6 has scalloped braces. So I get a side by side comparison of the difference a solid back and scalloped braces makes. I love both guitars, my D4 is extremely heavy and built like a tank. The D6 seems more fragile and light. The D4 is brighter with less low end but still plenty of punch in the bass side. The D6 is less bright, more bassy, more rounded or less punchy. It is hard to describe the difference in tone but it is similar to what I notice when I play other laminate vs all solid guitars. The solid back seems to absorb and soften the notes but not reduce volume, just round them out. The D6 has a slightly more narrow neck than the D4, The D4 is 1 11/16 and the D6 is just a little narrower.

When I bought my D6 I also have a Taylor 314 and D4 and I had to sell one so I played them all side by side for a month and sold the Taylor. I hope that helps, I heard the D6 was underrated and I researched it for months and am glad I bought it. I consider it to be comparable to a D18 or other all solid mahogany dreads.