[Pampero]

You would need a variable high pass filter and ideally, one with slopes selected to match your main speakers, outfitted with appropriate connectors, although you could probably get by with Phoenix connectors if space were an issue for the box. You'll need a power supply since this filter would be active, so you'll want a gain stage too. The easiest and really only way for a hack like me to obtain these features is to buy a true two way crossover.

When I speak of a high pass filter in my comments, I usually mean that it would be nice for us stereo guys if the sub manufacturer included the high pass (with appropriate capabilities) to match the low pass they usually give you.

Minimally, I'd want Linkwitz Riley, 12 and 24dB options, variable between, say 30Hz and 120Hz. Personally, I don't need a crossover that offers much more than 30 to 60Hz, but I can't imagine anybody building a crossover and not allowing for a higher frequency knee in the high pass section.
I'd also need balanced connections.

Since so few if any sub builders are offering that, it's down to one of a very few available stereo crossovers. There are no pro crossovers in my price range I'd trust to the job, either the cheap analog ones or the somewhat pricier digital crossovers I've looked at.

JL and Bryston seem the most appealing to me. Bryston builds the 10BSUB specifically for subs but I don't know much about it. I'd expect it to be excellent. The JL CR1 gets a lot of good reviews here and in the mags, same expectations.