Thanks for responding, Janey. In another thread (I've forgotten where), someone mentioned a particular weight (ply count) of cotton thread and how it has a tendency to stiffen a quilt. It got me thinking about the other factors that might come into play. When I was adjusting the tensions on the 99, I had the stitches perfectly balanced, but the tension was so high it felt unusually stiff for a single row of stitching. I re-adjusted, backing off considerably, until the thread was no longer sitting against the fabric (too loose) then increased the tension slightly. The result was neat, balanced stitches that contributed a minimum of stiffness to the test fabric.
I'm almost certain that longer stitches are more relaxed. This is important to me because I'm just about to embark on a simple quilt that I want to have as much drape as possible. I'm not willing to use silk batting for practical reasons, so I'd like every decision I make to be carefully considered. I'll gladly sacrifice a degree of durability so that the result is a supple quilt. Maybe long stitches under light tension is the answer.