Brian actually mentioned this to me when I bought my new actives. I haven't tried it yet (mainly because I'm returning them) but I've got my eye on another pair and I think I'll be giving that a shot.
An alternative is to use the #2 jack with a mating cable from phone plug to stereo RCA jacks. This is not a bad solution because a preamp needs some output impedance if it's driving cables. The raw LF output is less than 1ohm. The #2 jack with the resistors would have a 50R output impedance. This is better for a preamp.
BTW, you guys may not know that the 50R resistors at the #2 jack are Vishay zero capacitance, zero inductance resistors. They are custom made for each LF production run.