It depends very much on the subwoofer.
Rel (the old designs..not sure of new) are not really subwoofers but Sub Bass units and they work by boosting and adding on to the lower end of the speakers. the rels do not have a Low pass crossover..the sealed box design acts as a natural 24db d Crossover . Hence when you set a level you need to set it much lower than the 6dB point of the speaker and the natural slope (upper) of the REL is supposed to add to the natural slope of the speaker (Lower) and try to replicate the closest to a straight line (not easy). the positioning and the angle of the Sub is very critical and it does take a lot of time.
a corner /near corner placement is important for these as the subs amp is usually in the 100-150W unit and hence may not be able to give you the low bass it is capable off. a corner adds 9dB. the subs which can be placed away from corners usually have much higher powered amps.
eg in my speaker, which has a 44Hz 2dB point, the Sub is crossed over at 31Hz..and i just get to hear the 20-25hz notes. at a lower crossover Bass becomes much better, but the midrange suffers and above it i see a thickening and additional emphasis on the midrange.
Now if you had an active electronic crossover , your problem would have been easier and you could corssover atan exact freq.
What i suggest you do is place your sub at the place you sit with phase at 0 and walk around the room. find the place it sounds Loudest..that is your ideal position which takes your room into confidence.
once you find that, play around with phase/loudness and angle of placement to get the best possible integeration.
