And here lies the reason Audyssey pushed for "full range" to mean "< 40 Hz" instead of 80Hz, because the crossover frequency Audyssey found is not available on the Onkyos if it's less than 80 Hz. Very annoying.
Don't bother to set the crossover freqs before running Audyssey - it ignores them and does its own crossover estimates.
If your speakers are found to be full range, you can look up the spec, add 10-20 hz just to be safe, and use that value. Or if you just want to control the bass in the room by putting more into the sub, you can set the crossovers to 80 Hz. That way, your amp doesn't work too hard putting deep bass into your speakers, you get non-localizable deep bass from your sub, and all the speakers are guaranteed to be within Audyssey EQ frequency range.. Both strategies can work well, and it's easy to try both, and see if you prefer one or the other.
All my speakers show up as "full range". So their cutoff freqs are below 80 Hz, according to the Receiver's knowledge.
But I know that my speakers cutoffs are [probably about 40-80 Hz.
So: If I set a speaker' cutoff at 40, and it is really 80, then the receiver is sending the 40 Hz signal to the speaker, it has low response, and it is wasted power and lost bass; but if I set it to 80 and it is really 40, then the power is diverted by the reciever to the sub. I don't fully utilize the speaker's capability, but I don't loose any frequencies.
Tell me that Auddyssey is advanced enough to know that if power diverted from certain speakers due to their crossover settings, that the level of the signal sent to the sub is adjusted for its proper power level based on the known response curve for that subwoofer.
In which case, it would always be "safe" to set all cutoffs at 80 Hz so long as the speakers' is really lower than that and the sub's is higher than that. Any setting betyween 40 and 80 might be workable, but the accuracy of response would be unknown.
This seems like a question that there should be a clear cut answer to and should have been thought about in the initial design of Audyssey. The question could be resolved by a blanket statement, such as "If your receiver considers all speakers with a cutoff frequency of XX Hz and below as "full range", then set their crossover frequencies to XX Hz.
*Is* Audyssey compensating for the correct volume at each speaker after crossovers are introduced into the system response ? If you sent that low freq. to speaker A, it would be reproduced with a power B. But if it is sent to a sub, it is reproduced at some outher loudness level, unless compensated.
If not, it seems like another case of just goofing around and picking what sounds good !