I would go with the REL, simply because of its ability to mate with just about anything. It takes its input signal from the same line as your speakers, so it's "seeing" exactly what your speakers see in terms of the amplifier. The flexibility of the crossover is amazing. I'm using a REL Storm III to augment the bass on a pair of B&W Signature 800's--no slouches themselves in the bass department. I'm able to cross the REL over at 25 hz, so 98% of the time it's not even doing anything; but when it does, there's no question that it extends the lowest octave beautifully.
Subwoofers: Inexpensive Hsu vs REL
Does anyone feel the incremental dollars for a REL subwoofer versus an Hsu TN1220HO are well spent? I know the Hsu TN1220HO goes to 20Hz and is down only a little at 16, so it is not the depth. I would 'crossover' Merlin VSM-Milleniums at 40 or 50 Hz, so it is not a wide bandwidth involved. You don't need incredible detail below 50 Hz, so I imagine you have to work real hard to hear differences between subwoofers presuming they both roll off steep. The Hsu rolls off at -24db per octave, so you are not going to hear much above 50Hz, therefore it should not give its position away (e.g. 'directionality'). The Hsu fires toward the ceiling, not the floor, so you are not exciting the floor too much. You can get two (2) Hsu's for $1,500 - you can't come close to that with REL. Am I missing something? Has anyone heard both?