adding "slam" to 2-way speakers?

I've found that augmenting the output with a subwoofer in my larger basement room does indeed help with bass extension/weight/slam. However, if you find that the room is so large that it is bass frequencies ABOVE 80 Hz that ALSO need help, I would consider using a larger speaker with more/larger drivers. I set my sub's crossover at 35Hz, and yet it still does slightly augment the room response up to around 80Hz, and this works well for my floorstanding "MTM" speakers (each have two 5 inch woofers, and are tune-ported). In this large room, the speakers roll off very steeply below 40 Hz....but then, they're also nearly 7 feet out into a carpeted concrete room that is treated with several ASC traps. The room's volume is 4200 cubic feet............................To sum up: I highly recommend that you try a subwoofer, especially if it has many adjustable parameters, like crossover and phase. My opinion is that it is always possible to use a subwoofer to augment a speaker system, assuming the subwoofer is of adequate quality, and you are willing to spend the time it takes to adjust those parameters, and also expereiment with room placement and the direction the sub is "firing". And yes, two subs are better than one in large rooms, but two cost twice as much as one, and the end result is hardly ever "twice" as good, IMO.

My sub does that very well also, and it isn't a REL.

You are wanting to use a high-pass crossover on the Merlins? My sub's hi-pass crossover isn't transparent enough for that, and my philosophy goes against ever high-passing the main speakers anyway (extra line level circuitry, two more sets of brass RCA connectors in the signal's path, a whole other set of ic's, etc.). You should just get a sub, ANY SUB, and get started deciding what you like, and what you have time to do with your room/setup. It takes time, and thinking you'll solve all the problems ahead of time never got anybody anywhere. If you want another sub after trying the first one, they usually sell incredibly quickly. Or you might try the bass module with the Merlins, I hear they are pretty good.