Active Subwoofer Crossover

What are the differences between components like the JL Audio CR-1 and Bryson 10B-SUB active subwoofer crossover and the crossover built into most subs? Do they just provide more flexibility?

I'm not particularly technical, but I'll try to give the explanation I have experienced.  In some subs (like my JL F110 Fathom) there is only a low pass filter to roll off the upper frequency range of the sub, meaning you run your main speakers full-range and have to adjust the sub to integrate with them.  If your speakers go down fairly low in frequency response, this might be difficult to do without having some overlap in frequencies which also excites some room nodes.  I have been wrestling with this problem, as my speakers go down into the mid 30Hz range, and the lowest crossover point on my sub is 30Hz; I cannot get rid of a room node at about 50 Hz, even with the sub's equalization program, because the main speakers excite the room node a little by themselves, and even with a steep rolloff the sub's high frequency response accentuates that bump.  .An outboard crossover can take the full range signal from your preamp and split it at the chosen crossover frequency above the low frequency cutoff of your main speakers, so that your main speakers get a signal that is rolled off in the bass--this way they don't have to run full range and can have a crossover point that can be adjusted for room effects and better integration with the subwoofer.  Some speakers also may benefit from having not having to reproduce lower bass frequencies, at least from some of the reviews I have seen. 

As a potential drawback, there is the additional circuitry and interconnects, but a high quality unit like the Bryston is likely to add very little, if any, signature to the sound.  Also, in subs which have a high pass filter for the sub and a low pass filter for the main speakers, an outboard high quality active crossover might be better than the built-in crossovers in the sub, and result in better sound.  So yes, there is more flexibility, but you can play with crossover points and slopes and potentially get better sound as well.

Hope this makes sense.  Perhaps others with more technical knowledge can chime in.

rcprince. That was a clear and valid post. Sums it up very well...Thanks

So in theory, a external electronic crossover provides more flexibility and should allow a more seamless integration of the mains with the subwoofer? Does the external crossover take the place of the preamp or is everything still controlled via the preamp?

Generally you would run a pair of cables from your preamp (or, if you use a source direct to your amp, that source) into the active crossover, then cables from the crossover's outputs to your main amp and the subwoofer. The crossover would not take the preamp's place.

I added the CR-1 to my system. I tried the suggested 80 hertz crossover point and it was too lean. I've changed the crossover to 45 hertz and like the overall sound better. The biggest impact in my system is an overall cleaner/ more detailed sound with added space/ much so that I'm trying to adjust to what I'm hearing.
I've tried several different crossovers - some active and some passive.  the best sounding, and most flexible, i've found is the XTA DP-224.  one feature in particular that i found has made a significant difference is the ability to time delay the mains.  once the crossover and delays are dialed in, the integration between the sub and mains is absolutely seamless - better even than the vandersteen system, in which the HP filter and the sub amps are purpose built to provide a seamless interaction.
Glad to hear about your decision.  Keep experimenting with the crossover frequency.  80 Hz was really just a starting point, as you discovered.  Room nodes may also play a part in where you finally end up.
"one feature in particular that i found has made a significant difference is the ability to time delay the mains. once the crossover and delays are dialed in, the integration between the sub and mains is absolutely seamless".

"I’ve changed the crossover to 45 hertz and like the overall sound better".

+1 to both.

Also, a 45 Hz crossover means that bass can be separately adjusted if needed, say, for a particular recording, without unduly affecting the chestiness of male vocals or other lower-midrange sounds.

I've found more flexibility, with subwoofer crossovers in particular, to be key. Pretty difficult to overrate in an ideal setup, let alone in anything like a problem room or with unusual gear. 
Keep in mind, the only way your amps can ever communicate directly with your speakers is with an external crossover.  With an internal crosover, your amps communicate with the crossover.  With an external crossover, there is nothing between the amps and speakers except speaker wire.   
I thought setting the crossover to 80 hertz was considered optimal? How do you establish the correct phase?

I've been considering buying a cr-1 for my ayre/wilson sophia 2 system.   Are you pleases with your purchase?  Do you find it to be worth the money?
This also raises another possibility - Audyssey.  I've gotten great results integrating my subs with Ohm 100s using Audyssey in my main system,  but - as a practical matter - it means replacing the preamp with a pre-pro.  That's not going to win over too many 'philes, but I think it's easily worth it.  Crossing in the digital domain and EQing the FR makes the "hand-off" from subwoofer  to speaker impressively hard to detect.

However, if you use analog sources (I don't very often), it means either taking the subs off-line (direct mode) or subjecting your analog sources to ADC and DAC.  I'm okay with that, but YMMV.
Ricred1, sorry I overlooked your last post until now, my bad. Assuming you’re still looking for an answer, 80 Hz is really only a ’standard’ from the pov from a consensus of manufacturers...originally introduced to help consumers mix and match different brands of speakers, subs and receivers together back when surround sound was being introduced. It began as a midfi kind of thing. As you go up the food chain, the bigger working budgets allow the majority of manufacturers to solve the same possible integration problems in various ways...subs that cover higher frequencies, or satellites that go lower, etc. In that territory there is no standard, per se, since audio companies are perhaps more free to solve things for their own customers the way they see fit...but, some solutions may seem a bit more "universal" (or flexible) than others.

As for phase, there has never been a way to reliably identify the correct phase of a recording other than by ear. Whichever way sounds better, presumably, is the more correct choice for a preamp phase switch (if you have one), but it is to be applied to each recording. Some phase controls, like the ones for subs that offer a continuous range from 0-180 degrees, can be used to compensate for the difference in phase response at the lp between the sub(s) and the speakers, especially when the subs have to be placed and positioned in the room with tonal response as a top priority. This is dialed in by ear with respect to the lp, too. It can work pretty well, really...or at least if your subs are forward firing. Downward firing subs, or omni-directional as well as at least some of the more non-traditional sub designs (like transmission line, possibly) may or may not respond to a 0-180-phase control as well as a traditional forward-firing design. An ideal overall solution might be something like the DEQX, which can make linear the phase response of the entire speaker chain from top to bottom...very high performance, but also very expensive...and, of course, absolute phase of the recording would still be unaffected by that and independently still require a +or- phase switch. I don’t have such a switch for absolute phase, but haven’t really lost lots of sleep over it, I guess.

Hope this helps.
Thanks for the response. I don't have subs anymore and I sold my Aerial 7Ts. 
The Bryston will add the versatility and open the option of dabbling in an active full range set up down the road if you get the notion. Marchand is one other good make and offers tube and SS options. MiniDSP is another worth investigating.
I'm getting Monitor Audio PL-500 and hope I don't feel the need for subs. 
I do not find that subs are always "necessary" meaning I don’t always have an irresistible "need" for them with certain full range speakers that do quite well covering bases effectively. That said while knowing full well what can yet be obtained by adding them makes the urge to add them that much stronger. It could be argued that desert is not necessary but its results are obvious while enjoying that big piece of double layer cake along with your favorite ice cream.

There is an active add running for the Bryston here on Audiogon. 
I received my Monitor Audio PL500s and I'm so excited that I don't "need" subs. Yes, subs made a difference with my Aerial 7Ts, but there is something about a "full range" speaker in a relatively big room that a smaller speaker with subs just can't duplicate. I have so much respect for the Aerial 7Ts, but I realize my room is just to big for them.
It sounds like you could use some subs!

I reckon subs in a way are like that old chicken and egg adage. In that they too will continue being debated well on into history.

I have full range 3 way speakers of lesser volume but quite adequate for the room they reside in. Bass they emit is exceptional, not a need for subs, really. Until I take a short walk and encounter some weak bass. Modes anger me often to the point of agitation. My pressure rises steam exits my ears and my left eye starts twitching violently causing me to miss half of the game. I’m probably one of the few that needs subs.