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.
You could always look for a used Pass Labs XVR1, has both SE and XLR input/output w/ dedicated power supply. Here's the one-sheet and manual.

Terry Cain loaned me his for a month or so when I first got my IM-Ben double-horns w/ matching dual subwoofers. The XVR1 was as close to "transparent" as you could get.

Depending on whether you are a tube or SS fanatic, digital or analog, it really doesn't get much better than that.
Dear Pedrillo: IMHO I think that you already owned through your ML sub and I think too that its high pass filter is really good.

Why do you think you need an additional external crossover?

Regards and enjoy the music.
The one thing it doesn't have is balanced inputs and outputs. I like how my sims (pre & power amps) sound in balanced mode.
ARC, great sound and ability to get support in the usa
NHT's X2 filter I'm pretty sure does not provide 40Hz as an option. Unless your Merlins use 10" driver or larger, I wouldn't let them see anything near 40Hz. Request some distortion data on your speakers and you'll have a better idea where the crossover point shoule be.

I believe the DEQX is digital, so there will be an A->D/D->A conversion involved. Probably not necessary for just integrating a sub.

I believe the Bryston 10B comes in two flavors -- one specifically for subs. I'm a big fan of Bryston, but I see no reason to spend that kind of money when the NHT X2 is available.

I have no clue on the other's you mentioned.

Another option would be a Tact unit.
The one thing it doesn't have is balanced inputs and outputs. I like how my sims (pre & power amps) sound in balanced mode.
I would enjoin you to consider a second subwoof instead of an external electronic xover. Sonically, there will be little if any perceptible difference between balanced & SE at low frequencies. There will be perceptible & significant difference b/ween one & 2 subwoofs.

Anyway, amongst the units you list, the Marchand is the usual culprit, well made, good sound, & dependable.
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.