@ryanosaur
Is this recommendation of "

**always start 1 octave above bottom frequency**" independent of the roll-off rate of slope ? Or is this based on a 12db/octave slope ?

So in this example, his speaker's frequency response is 53Hz - 25Khz

If this is sloping at 12db/octave, then yes a crossover frequency of 1 Octave from bottom(ie 106Hz ), rounded to 100hz will ensure , the bass o/p is atleast 4 times lower (ie -12db) at 53Hz , than what it is at the XO(100hz). So XO of 100hz is fine here.

But if the roll-of slope is 24db/octave, wouldnt half an octave(ie 79.5 rounded to 80hz) for XO be sufficient, to get the same effect (ie -12db at 53Hz) ? . ie Cross-over frequency of 80z will still reduce o/p by the same factor of 4 at 53Hz and get away with a lower XO?

Reason am bringing this up..is my ML 35XTi has a similar frequency range(50hz- 25Khz) and exhibit a 24db/octave slope (and not 12db/octave) from Audioholics measurement. Yes, I'd have usually set it up with a 100hz XO , but looking at the steeper slope I reached out to MartinLogan on the crossover..,and they recommended only half-an-octave above the bottom most frequency (ie 80hz) and not a full Octave(100hz). This was also the recommendation from SVS(ie 80hz) for my speaker with their Sub ..and infact recommending going down to 60hz for subwoofer's LPF in a 2 channel pre-amp setup for my ML bookshelves, given their Sub's amp fixed at 24 db/Octave.

The other potential concern I have with 100+ hz for XO is higher localization than a lower one(like 80hz). In reality localization may not occur until we reach higher frequencies(120+)..,but thats the other thing I had in mind when debating between half-an-octave vs full octave.

Also, the lower end of male voice starts at about

80-100hz. So isnt there a risk of redirecting lower end of male voice to sub-woofer, with a XO of 100+ Hz with possible localization or lack of clarity?

I'm really good with starting at 80hz for XO.., but not too sure starting at a full octave(ie double the bottom frequency) for all cases independent of the roll-off rate and speaker.

Your thoughts ?