I don't know how to set a pre fader send in Reaper as I don't use this DAW, but it should be explained in the Two Notes link I gave you earlier.
About the phasing issue I think it's normal, as I think Kalthallen's cabs are not phase aligned (Skaldir, am I right ?)
So it's like in real world, some combinations will work, some will give you phase mess, depending on each mic position.
Just try different combinations, flip polarity if needed and that's it !
If a polarity flip don't work, you can try plugins like Little Labs IBP (UAD only) or Voxengo PHA979 which allow you to tweak phase from 0° to 360°.