Josh, dcc track voltage is actually 13v and the output functions on most decoder out put the track voltage, so 13v - 1.5v = 11.5v; 11.5/.015=767.
Try 767 ohm resistor to start and if it is too bright increase by 50 ohms, if it is too dull, decrease to 700 ohms.
Let me know how this works for ya.
I haven't used the DA-SR's yet and have been tempted to, most of the time I use TCS A6X's in the genesis units, with the exception of the units that come with the dcc-ready board built in, then I use the NCE D13SRJ 4 function.
The reason behind the A6X usage is that they have a built in voltage regulator and output 1.5v just for the Athearn bulbs. So all you need to do is solder the original 4 points for pickup, 3 points for forward and rear lights, and then 2 more points for the ditch lights, as the 3rd point is shared with the + from the forward and rear lights. I get mine from dccroundhouse.com for $22 @ plus shipping.