adding "slam" to 2-way speakers?

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

hey carl_eber, what kind of sub do you use? i've heard great things re: REL & Vandy, but i'm trying to keep my options open. fwiw: i don't want a sub to simply 'fill in' info that my mains cannot produce--i want a sub to reproduce the same bass info in a non-directional manner so that a) i cannot discern where the added bass is coming from b) additional weight & scale is provided to the music thanks to the subs reproducing the same freq. as the mains (and implicit in this is the requirement that the sound of my mains is not affected--the merlins are incredibly coherent) (and given the higher freq coverage of a sub, should i look exclusively at front firing subs like M&K?)

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.