Some of the reasons for subs sounding boomy.
1. Most of the subs have too much gain which causes this.
2. Also sometimes break in is required before the sub settles down a bit.
3. Your room has its own bass nodes. Try corner traps.
4. Play with the crossover, phase and gain of the sub. How have you connected it? Via lfe or direct?
If it's via lfe, your subs crossover won't work and you are dependent on the receivers crossover. What's the crossover set too? Some of these big subs don't like to be crossed over high. Your bnw doesn't go low at all. It's just mid bass. So Ideally you should cross it over at 60 or 80 and play with phase to integrate it well. However if you placed it where a room node is getting hit nothing will help you unless u damp it with bass traps.
5. Ported or sealed or passive radiator don't matter. The same design can sound fantastic or rubbish depending on the implementation.
6. Placement is critical. The sub has to couple with the room. Even few inches towards or away from the back all or corner change everything. You will have to play around with his. Read the REL subs manual online. They explain this well.
Why add a sub? It's either for better presence or ambience or to set the bass foundation.
Some subs tend to be smoother, some hit harder, some are boom boom and so on. The smoother ones give a better ambience and presence but you won't feel them. The bnw sub will mostly be in this category though will boom a bit as it doesn't use a very high powered amp.
The hard hitters set the bass foundation on which your main speakers have to draw the portrait. They can sound stunning if done well but they will over power your sytem if you try and set the sub to a higher crossover. I believe this is what you are experiencing. Imagine feeling king kongs steps rather than just hearing a boom boom when he walks. Believe it or not. Some of these hard hitters have to be crossed at 40hz or they will go awry. The hard hitters usually need vary large floorstanders like say a bnw 802.
The boom boom subs are the cheapy or poor designed ones. They cannot do presence nor set the foundation. It is almost a function of wrong design though a well designed sub that runs out of power will also boom.
I usually say if you have weaker main speakers go with an ambience sub.