Any tips on this? it seems to be the biggest issue with mine, I either end up with way too much (when combining the sub with L / R) or way too little (with the sub on it's own channel).
I'd start with what you're telling DL to correct. What do the raw response measurements of your left and right midbass and your sub look like (with your selected crossover engaged, and also without them)? Now, when you play them all together what do you measure in REW? Maybe there's something going on there that needs to be addressed before you tell DL to go to work.