Hi Marlboro
I started to notice this first when I rewired a MTM from parallel to series using the same drivers in the same boxes.trying to save money on the xover designs I was playing with.The parallel just sounded better and i believe that is of better control the amp has over the drive units wired this way.I took this further with four 16 Ohm drivers.Adjusted for the same SPL level the four parallel just where somehow cleaner.Cant technically explain it but there is nothing wrong with my ears.
Cheers
Sunny