Sub/Speaker Crossover?

Is there an accepted "rule of thumb" when it comes to where to set the crossover between subs and speakers in two-channel?

I have Spendors that go down to around 45 and have always thought it best to set the sub crossover to 50 -- just before the Spendor starts giving out. But I have also read where some set the crossover for similar monitor speakers way up at 100! Or even lower, say 35 or 40.

It seems to me that there should be -- regardless of speaker/sub manufacturers -- a reasoned and correct crossover target. (For the record, I use ACI subs with my SP1/2s and SP100s.)

Thanks to any and all who can educate me. Just seems like, for this one, there should be a "right" and "wrong" approach.
I like to let the speakers run full range and cross the sub over around the roll off point of the mains. This way you don't get the sonic signature of the xover into the mains. You should of course listen yourself for the best bass in your room.
Do your Spendors go down to YOUR room? Probably not. I would cross them over at 60. I also agree to listen for yourself and determine the best crossover point for your speakers and YOUR room.
With a sub, your placement doesn't have to be next to the mains. You may find that because you can move the sub to an adjacent wall your perceived bass may improve a lot (in the mains). I'd try 60hz to start, with the sub on an adjacent wall. Unfortunately it eventually comes down to trial and error. Keeping the crossover low will allow maximum bass to come from the mains, which in conjuction with a sub (a second source of low frequencies) may help even out room nodes. That works even better with stereo subs.
I use SP-1s and S 100s with a pair of REL Stadium subs, I use 25 Hz. I would never go above 40, 60 is for HT set ups. I let mine go full range whether I am using the RELs or the Hsu I use in my HT or the Nelson Reed 1204s.
I cross over my mains into my sub at 80Hz. I tried everything from 50Hz to 100Hz, and found that 80Hz resulted in the smoothest integration. However, I cannot say that 80Hz would necessarily work best for you, since, after setting up several mains/sub systems in several rooms, I came to the conclusion that the choice of crossover frequency is very system/room dependent.

Another thing to keep in mind is that the perception of of the "best" crossover frequency will be influenced by other parameters of subwoofer setup, such as placement, phase, level, and time alignment. All of those parameters interact psychoacoustically, so it is often challenging to identify which parameter needs to be adjusted. In my case, it was a frustrating process that took several full days of experimentation distributed over a couple months.

Having offered those words of discouragement, I will say that it is totally worth it, so hang in there!
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Wow. See what I mean? :)

Thanks for all the input, folks. I used to have the REL and know they are designed for just the low stuff below what any monitor can reach -- hence the setting in the 20s.

I've always just fiddled with my subs until the bass sounds just right in Jimmy Smith's excellent "Dot Come Blues." I trust my ears. But it does seem like reasoned audiophiles have very different "rules of thumb" when it comes to this topic!

Keep in mind that a good low setting won't draw attention to your sub.Done right,nobody should know its on.
The rule of thumb is to set the crossover one octave above the F3 of the main speakers. This much overlap is needed to get a smooth transition. If your speakers are -3 dB at 45Hz, then you should set the crossover to the sub at 90Hz. No matter where the crossover, integration is not easy. You can try to do it by ear, but test signals and an SPL meter will get you there faster and easier.
I've always just fiddled with my subs until the bass sounds just right in Jimmy Smith's excellent "Dot Come Blues." I trust my ears.

Jd...You hit the nail on the head my man.Trust your ears,after all,you are the one listening to your system.
After years of searching for the "correct" crossover frequency I have realized that it doesn't exist. The best frequency depends on the type of music you are playing. The crossover should have an easily adjustable crossover frequency (a knob) and you should diddle with it just as you might do with a tone control.
As usual, Bob has hit the IMHO critical points. An illustration from Verity Audio. The Parsifal monitor is down 3 db at 55 hz. The x-over in the Encore woofer is set at 150 hz. Obviously stereo cabinets are required. one more reason to cross high. Unless you use bass busters, there's a very good chance that your worst room mode will be a hump between 80 or so and 125 or so hertz. Properly placed Subs - especially of the drc type - can adress this issue

good luck

For Movie - 80.
For Music - 0-5 below your mains capacity.