In reply to:
Someone on the other hand using much larger speakers might(mistakenly in my view)want to set the crossover even lower than 80Hz for fear of "wasting" some of the bass that their speakers were capable of.
John, you bring this up all the time but you have a "mistaken" view of people's perception. Granted some newbies have the question, "well why did i buy towers if i'm not going to use their bass drivers anyway?" and they should be given both reasons so they can decide for themselves.
Just to be clear now, there is no fear one is losing or wasting the bass of our towers. People, such as myself, may not like the overbearing sound of a sub playing notes in the higher bass area. I find it too boomy regardless of gain settings and if i lower the gain, i would have to turn it back up again for HT. Using the sub takes away the whole tightness of bass i was searching for in a speaker in the first place.
My personal reasoning for using a cross on the sub at 45Hz-50Hz and allowing my M60s to play as 'large' is to use the bass of the M60s for music while keeping the subwoofer for the truly low end musical bass notes (what the subwoofer is really designed for) and yet still having it add plenty of explosion oomph during HT.
As long as one mixes the crossover so the sound meshes together well, then it doesn't matter whether the THX suggestion of 80Hz or whether the cross is 50Hz, 40Hz, 120Hz, etc.
There is no 'mistake' in doing so.
As for the idea that Alan mentioned in regards to lessening the burden on one's amp/receiver, as long as the unit is robust enough, the difference is negligible if at all. I certainly have not heard any significant change in using the 80Hz vs. a 50Hz cross with my M60s. Certainly nothing that is so pleasing as to override the awful boom bass i also have to put up with in such a configuration.
If you try it out, it really does work nicely.
For those who have the M22s or another speaker with a far lesser bass extension or perhaps prefer a heavier low end sound than what the M60s can put out, then by all means, use the 80Hz or higher values. In this instance, the sub will provide more benefit than disadvantangeous boomyness.
