[Ron Whitaker] “So, if I simply plan on using the DR-40’s phantom power, which setting (Off, On-flat and On-roll-off) do I set the power supply to?”
You’re not going to like my answer but it depends on the circumstances and your talent. All I can suggest is to experiment when you use it and see what sounds best for each setup.
From the manual: the switch is an 80 Hz high-pass filter that provides easy switching from a flat frequency response to a low-end roll-off.
“Am I also correct to assume that if I’ll be using phantom power, I won’t need to put any batteries in the battery compartment?”
Correct. From the manual: The AT803 requires 11-52V DC phantom power, or a 1.5V AA battery for operation.
A battery need not be in place for phantom power operation