To expand a bit further on things beyond desertdome's post:
What your first post describes is common audiophile rules of thumb or more accurately folklore. Most of this comes from plug-n-pray experimentation and anecdotal cases which might be common, but are certainly not a rule.
Remember that most such recommendations originate to a time when measurements were a black art and full range speakers ruled. The mindset of recommending as low a crossover as possible comes from a past reality where enthusiasts had at most 3 dials to play with when dialing in a sub woofer (Level, XO, and phase). They were generally trying to combine with their main speakers which were still running full range with no high pass crossover, and the speakers were voiced to sound balanced on their own, no subwoofer needed. The recommendations tended to revolve around theoretical ideals of speaker behavior which is only observed outdoors or in a textbook. In past times with 2ch systems there also were very few who had expectations of dynamic playback levels anywhere near what we now know is possible and expect in modern home theater and 2ch systems.
If you did get lucky and achieve a very nice extension to the bottom of most bookshelf speakers, it sounded a little thick because the factory voicing accounted for a lack of deep bass which you just added back in. There is also the very significant factor of in-room response. If you limit the upper response to ~40Hz, you have a little better chance of not having a big modal peak in the upper bass range which can be very audibly offensive. One of the most significant keys to utilizing a higher crossover frequency is smooth in-room frequency response. In your own case, the use of the AntiMode immediately provides you options those just twisting a few knobs don't have.
Finally we get to the behavior and capability of various speakers. Today you can find everything from compact speakers extending to near 30Hz and monstrous speakers intended for use with subwoofers with huge output to their low end cut off of 60-80Hz. The appropriate crossover implementation for each will be very different, and will differ still on the application (room and playback expectations). Most of the simple crossover rules of thumb stem from a blissful & idealistic approach which has little basis in reality. The approach of crossing at least an octave above the lower extension limit of a speaker comes from such an ideal view. It can be a workable approach, and for speakers with deeper extension, even desirable, but is certainly not a requirement.
Here are a few guidelines off the cuff:
- Are you able to high pass your main speakers?
- If yes to #1, choose a crossover high enough for desired playback capabilities and above any major holes or peaks observed in the response at your seat.
- If no to #1, make your best estimate at how low the speakers extend in room and experiment with a subwoofer crossover near that frequency.
- Do you have EQ available on your subwoofer?
- If yes to #4, you are more flexible with higher crossover frequencies if desired, but still subject to room response limitations.
- Either you can see the measured EQ'd result or you make a best guess and are left to listen to check how effective the correction was.
- If no to #4, measurements in room can be even more valuable to find the best subwoofer location and possible crossover frequency. If not, it's a plug-n-play effort.
In the end the appropriate effort and approach to executing a sub-speaker crossover depends greatly on the options and adjustments available to you.