There's a difference between adding "bass weight" and "adding bass extension". It would appear that the Rel is more suited to adding "weight" to speakers that may suffer from a lack of output on the bottom end. In effect, the REL may not really be extending the bass all that much, it is simply adding more of what is already there. That's why it is easier to blend. If the system sounded lean, it's quite possible that the addition of the REL could drastically increase the solidity of sound reproduced WITHOUT massively exciting other nodes in the room that are lower in frequency.
The Outlaw looks to be a much better design if one is looking for quantity and extension, but it could be quite lacking in quality. That is, one can achieve very high peaks and great extension by using a driver that is very hard to control ( giant impedance peak at resonance ) and tuning the system for a lot of "false" aka "undamped" bass output. The end results is GOBS of bass, but quite lacking in transient response i.e. detail and definition. Without looking at the actual test results and knowing how the tests were performed, i'm basically talking gibberish though.
Other than that, i am of the persuasion that some spec's can provide you with a very good idea of what to expect of a component. Maybe not the whole story, but at least a good portion of it. Sean