I think you should be able to set 40, 50, 60, and 70Hz too, my 875 can.

Set it at about 10Hz higher than your speaker's lowest rated frequency. This is where the specs sheet is useful, if you don't have it you have to use test tones (with crossover turned off) and check the lowest tone it can produce.

And yes, anything below 80Hz will be sent to the subwoofer. If you speakers can handle down to 80Hz, try not to set higher or else your subwoofer is directional already. But the advantage of this, the bass thump will be more defined and sharper. So it's still personal preference, as the great philosopher once said, "the code is more of a guideline than actual rule" (Captain Hector Barbossa).

Oh, you mean the Low-Pass Filter for the LFE? That's a different setting, kind of the opposite to the crossover actually.

This setting is only for the LFE channel from movies, the .1 in 5.1 or 7.1 tracks. This extra channel is actually created based on the same idea/concept of crossover, but it was already done for you first-hand by the movie engineers in the mixing studio, putting the low frequency in a separate dedicated .1 track . Some of the engineers might put the LFE too high (though not actually wrong, just his preference), maybe let's say 100Hz. So, the purpose of the LPF in Onkyo, is to prevent the high frequency from the LFE track from being sent to the subwoofer.  Meaning if you set the LPF at 80Hz, the frequency range between 80-100Hz of the LFE track will be filtered.

If you just want to just hear it just the way the movie sound engineer intended, just put it as high as possible. But, as I said, higher frequency will make the subwoofer directional but with sharper and defined bass thumps. So again, it comes back to your own listening preference. Only to watch out, don't set it too higher than your subwoofer's high frequency capability.

Hope I don't confuse it further.

Using a Y-cable into both the L & R line-level inputs on your sub should give you an approx. +2dB gain in output from your sub (this is what i noticed in my case, when i went from a single el-cheapo RCA cable to a Monster subwoofer cable with a Y-adapter). I also found that the sub's sound then appeared to become a little more 'boomy' (maybe it's just my imagination).

What i always do whenever i change my setup is to re-run Audyssey calibration (after performing the usual steps to calibrate my subwoofer prior to running Audyssey). Please see this link - Audyssey Setup Guide.

0 is facing u
180 facing ur butt

I think the LPF (Low pass filter) should be set to the max possible (120 hz if i'm correct). If you set it to 80 Hz, all the frequency between 80 - 120 hz from the LFE channel will be filter off. The 80 hz recommendation from THX is for the other 5 channels not for the LFE channel which mean any frequency below 80 hz from the 5 channels will be send to the subwoofer plus the original LFE channel. The LPF setting is meant for those people who had subwoofer humming problem which they can minimize by reducing the LPF frequency. If you don't have this problem, set it to the max possible.

I think he is talking about the LPF setting in the amp, not the setting at the back of the sub.
Swine, can u confirm?

Set the LPF to the highest possible for the LFE channel (note LFE channel only, not the other 5 F/C/S channel). On the sub, set it to the highest possible, let the amp control this. If you face some distortion, try to reduce the dB level for the subwoofer. Did you calibrate your sub with SPL meter or you just use Audyssey?

Yes, set the LPF to 120 hz on the AVR and the sub. LFE channel usually coded at 120 Hz and below. This mean all the LFE signal will be channel to the sub. If you set at 80 Hz, that mean from 80 to 120 Hz signal from the LFE will be filter away. Now don't confuse this with the 80 Hz setting for the other 5 channel (Front/Center/Surround).  You can use THX recommended setting of 80 Hz. You can tone down the subwoofer by reducing the setting by a few dB if it's too bassy. Another way is to move the sub away from the wall.

If you set the LPF to 80 hz on the sub, any frequency above 80 hz will be gone including those from LFE channel. The setting at the back of the sub will not tell the AVR to direct the frequency between 80 - 120 to the front. The lpf filter on the back of the sub will be the last filter that the sound signal will pass thru. So you should set it to the max possible. If you can by pass it, better still. Let your AVR control your frequency setting. Not sure if you get what I meant here. Let me recap : there are 3 setting here. The first 2 settings are in the AVR which is the LPF for the LFE channel and the freq cutoff for the front/center/surround channel. Then there is another manual setting for the LPF on the back of the sub. The correct setting if you want to follow THX recommendation should be :

LPF for LFE Channel : Set to 120 Hz
Freq cutoff for Front/Center/Surround : Set to 80 Hz
LPF setting at the back of the sub : Set to max possible (120 Hz)

If u follow this setting, the full freq range from the LFE channel will be send to the sub together with the signal from F/C/S which is below 80Hz. When this signal reach the sub, it will be able to pass thru all. If u set at the back of the sub to 80 Hz, those signal between 80 -120 Hz from the LFE channel will be filter away. Hope this is clear enough.