Which external cross-over x-over to consider?

I would like to use an active x-over with balanced and single ended i/outputs between pre and power amps for a sub(Martin L. Depth) and my merlins, crossing over at around 40hz.
I never realized that there are a bunch of extrenal x-overs to choose from. Which ones would be of higher quality?
Here are some I have found, but I am sure there are many more:
Marchand's xm126 & xm-44,
nht's x1 & x2,
Bryston's 10b,
Mark levinson LNC,
deqx pdc-2.6,
Audio Research EC21.
If there are others worthy of looking at let's hear about them.
I use the Ashly 3.24 CL with great results. I use it with Final Sound 1000i's and two subs.
Dear Pedrillo: I agree with Gregm about bass performance through balanced or unbalanced way.

IMHO I think that if you " insist " on the balanced connection it can/could be better to change the sub, Velodyne is a good alternative and comes with balance connection. Btw, like Gregm posted, two Velodynes can delivery magic on your system.

Regards and enjoy the music.
The Marchand products are great. Their cosmetics are not so great, but the circuit design is straight forward and works very well.

We manufacture a [http://www.rivesaudio.com/PARC/PARCframe-sub.html]sub-PARC[/url], which is a sub crossover AND amplifier. Obviously this is overkill if you already have the amp, but if you have a passive sub this is really an incredible product.