So here’s my two cents: REL subs are wonderful and generally integrate into a 2 channel hifi system easier/more seamlessly than other subs. HOWEVER, they accomplish this by tapping into speaker level. Because my amp (Raven Audio Osprey) has a high pass filter at 80 hz, Using the HPF relieves my amp of having to reproduce sub bass frequencies (which effectively triples the headroom) and relieves my speakers (Raven Audio corvus reference monitors) of being encumbered by sub bass frequencies.
So, even though the RELs sound fantastic and integrate so well, the dual B&W’s I settled on that connect at line level work better overall in my setup. They took a LOT longer and way more effort to “dial-in” (in terms of position, gain, and crossover frequency), but now that I’ve got them in the sweet spot, I’m very very happy with them.
Finally, I 100% agree that stereo, SEALED subs are the way to go. The sound is tight and responsive and fills the whole room in a completely “de-localized” way.
good luck!