Should towers be set to small and crossed over at 40hz or 80hz when running a sub? Seems to be a lot of conflicting info out there about whether towers should be run large or small.
The Large/small/crossover settings are kind of confusing.
Let's first cover/review what these parameters mean
(at least how I understand them ... please correct me if I have something wrong):
LARGE/SMALL
The large/small setting tells the receiver or processor if you want to send the speaker a full range signal (i.e. ignore the crossover setting) or If you want to redirect frequencies lower then the set crossover frequency for that speaker to the sub-out. So, Large = full range, Small = anything below the crossover frequency is not played on the speaker but sent to the sub.
Note: If your speakers are all set to large the sub-out may not send anything unless your playing a movie with an LFE channel (Low Frequency Effects channel)
SUB MODE
There is another parameter my 8805 calls Subwoofer mode. I can set this to either LFE or LFE+Main.
LFE works exactly as stated above: the sub will play the LFE channel and any low-end content redirected from speakers which are set to small. The Large speakers handle their own low-end.
In LFE+Main mode the subs will in addition be sent the low-end content of the Large speakers. ie. the frequencies below the set crossover of a large speaker will be sent to both the speaker and the sub. OR stated another way, the sub gets the low frequency content from all the speakers. (large speakers still play the full-range).
So, after all that we can talk about why you might want to set a speaker to large or small.
Obviously, if you have a small speaker which doesn't go low you want to set it to small and let the sub handle the frequencies it can't. 80Hz is a good choice because below that it's hard to localize where a sound is coming from and most small speakers can play that low.
But what if you didn't have a sub? In that case you might consider setting even a small speaker to large so the full frequency range gets sent to it. (Most receivers will do this automatically)
If you have floor standing speakers which can go fairly low ... say 30 Hz but you have a sub which can go even lower you might want to set that speaker to small with a fairly low crossover ... like 40Hz. Alternately you could set your subMode to LFE+main and play both (adjusting the sub knob for a smooth transition). The receiver also has a LPF (low pass filter) setting which you can set on the LFE.
Another reason you might set tower speakers to small is because you're trying to drive them with a marginal receiver. By off loading everything below 80Hz (just to pick a number) to the sub you might get a better cleaner performance and be able to crank it (where the receiver alone would run out of juice).
I'm sure there are other examples out there but each setup is unique and it's best if you understand these parameters to make the best trade-off for your situation.
