I thought I'd post this mini-review only because I recently spent several weeks trying to get information about this crossover without any luck and ultimately took the plunge with very little information to go on. Fortunately, I'm pretty happy with my purchase, but in case there is anyone else looking for some user experience regarding Marchand's variable frequency active crossover, the XM-66, hopefully you'll find this useful.
For the last several years I've used a 2 channel dbx 223s crossover in my 2.1 home stereo rig to relieve my power amp of the low frequencies. It was pretty cheap, about $150 at the time of purchase, and I felt it didn't noticably color the tone, but when I would compare the sound through my speakers to a system without the crossover in the signal path, I could tell it was adding some low level but nonetheless audible noise. I talked to tech support at Harmon Kardon at one point, and they advised me that the noise was a result of the dbx 223s being designed for a +4dbu (i.e. pro audio) nominal signal instead of a lower nominal signal typically found in home stereos. Although who knows, it may have still been audible at higher line levels as well.
A few months ago I began looking for crossover to replace my dbx. First, I wanted it to be analog (there are some folks on this forum who swear by the miniDSP -- if it works for you, that's great, but the heart wants what the heart wants). Second, I wanted it to be dead silent, totally transparent, with 4th order low and high pass. Third, I wanted it to have damping control to help me dial it in, and I preferred a variable frequency, which ruled out Marchand XM9. I didn't want to spend more than about 600, although I ultimately raised my price cap a couple hundred to 850. Finally, I wanted it to either support balanced and unbalanced, or else be upgradable to balanced (my rig is currently balanced from DAC to preamp, and then unbalanced from preamp down to the power amp, although I antitipate eventually getting a balanced amp and running balanced all the way through).
If you've been looking into [analog] active crossovers at all, you've probably noticed that there are a bunch of pro crossovers priced between 100 and 300, and then most crossovers designed for home stereo. The problem with these crossovers is that all of them have the same design issue that the Harmon Kardon techs mentioned to me -- they are designed for hot pro-audio signal levels. They might claim particular SNR and particular distorion specs, but those specs are for +4dbu. Typical home audio nominal signals are -10dbV, which is about 12dB softer, and at that level the distortion and SNR are a lot worse.
Home audio crossovers, on the other hand, are significantly more expensive. The cheapest I could find was the Hsu crossover, priced around 500, and they go up from there. Most are well over 1000. Eventually I settled on the Marchand XM66 because of its variable frequency adjustments. I couldn't find any user or pro reviews, although there were some very positive reviews of the XM44, and Phil Marchard wrote to me that he couldn't hear any autible difference between the XM44 and the XM66 (althoug he acknowledged that in theory one might expect fixed frequency modules to outperform a variable circuit).
Now that I've had the XM66 in my rig for about a month, I feel comfortable sharing some thoughts. First, it's built like a tank! Second, it is, indeed totally transparent to my ears. I have the unit set to 50Hz (after some experimentation to integrate my sub). When I compare music that does not have any bass with the crossover in the signal path to the same music without the crossover, I cannot detect any difference. With bass, integration between speakers and sub is pretty darn good. I'm quite content with the damping, phase 0/180, and frequency adjustments. Undoubtedly if I had seperate adjustments for low pass and high pass filter frequencies I could probably achieve slightly better integration, and this might be important for some, but I don't need this level of flexibility. Lastly, Phil Marchand is very helpful, and answered several of my questions.
The one negative I have to share is that Marchand has unfortunatley stopped selling crossovers as kits. But it's hard to blame him! I doubt he made any profit on them.