QTR-Quadtone RIP

I use QTR 2.5.1.0 with an Epson 2200 and matte paper. I've printed so 
far with 280 dpi a 3018x2010 pixel file printed on a letter size paper 
to about 10.76x7.17" and noticed a slight loss of contrast as compared 
to using the Epson driver. To compensate for this loss of contrast I've 
applied and S-curve with 3 points on the curve: 64/58, 128/128 and 
192/198. So far so good.

 
When I print with 305 dpi I notice a greater loss of contrast that 
would require a more aggressive S-curve. I would expect the same 
tonality across a wide range of dpi's. What is happening? Would 
switching to the newer QTR 2.5.2.1 resolve this problem?