[docmd2010]

Congrats on your new REL's.

I own a pair of 25's and have them very well integrated with my loudspeakers. Here's what I would do:
1. Once you have these plugged in, continuously play a song with some bass on repeat. It doesn't need to be loud, very low level is ok, as all you need is a little bit of driver excursion, as the components of the driver just need to loosen up. I would let them break in like such for probably a week. You can still listen to music as normal, but note that the performance of your REL's will improve after a week.
2. The REL manuals give several positions of stereo placement but i found that the subs were placed best at the same position as the speaker, but immediately lateral to them. You will need to crossover the subs depending on your loudspeaker. It took me about 2 months of fine tuning (small movements of the sub and playing with crossovers) to get the 25's fully dialed in.

The end result:

An amazing low end, which really improved the midrange presentation. When properly dialed in, my 25's disappeared. When sitting in my sweet spot, I had no idea where the music was coming from. Both my speaker and 25's were statues in the room.