But, now, how about this:
Do movies put content lower than 30hz in the front L+R channels, or do they just reserve that content for the LFE channel?
I haven't done any kind of exhaustive analysis but there is no reason a movie soundtrack couldn't/wouldn't put sub 30 Hz content in the regular channels. The LFE channel is for the 'effects' - like the loud explosion you weren't expecting that suddenly scares you out of your seat. But, any music that accompanies the soundtrack may have deep bass as well.
My front L+R speakers (BTW they are Polk) say they go down to 30hz.
My sub says it goes down to 20hz.
So, if I have the front L+R set at full range, that means they are getting 20hz tones, but not playing them, because the speaker only goes down to 30 hz.
Speakers (and the xover circuitry in a receiver too) don't just stop playing at a specific frequency. It gradually rolls off so the speakers may be playing the 20 Hz tone but it's so low in amplitude compared to the higher frequencies within its range that the effect is muted.
Configuration gets tricky with speakers that include a 'sub' as part of the cabinet because there are a few different designs and you really have to think about signal routing and which speaker(s) you want to do the work:
Case 1: If the speakers have a separate external input for the subwoofer part, then it can be considered a separate sub for all intents and purposes. In that case, you'd connect an RCA cable from the receiver's sub pre-out to the sub in the cabinet and set subwoofer=Yes in the receiver setup menus. You'd set all channels to a xover frequency you want (80 Hz is often best) and all the bass below that point, including the LFE channel, gets sent to the sub in the speaker cabinet.
If you also want additional external subs, you'd either use a splitter from the receiver's sub pre-out or daisy chain the subs together. In either case, all subs (including the built-in) will play all channel's bass and LFE.
Case 2: The speaker has a built-in 'sub' but no external input - just normal speaker wire connections. It sounds like this is your case because you said the sub has an internal xover of 80 Hz.
You'd want the fronts with the sub set to full range and let the speaker itself hand off sub 80 Hz bass to the internal sub. Subwoofer=no in the receiver setup.
Using additional subs
Again with using additional external subs, you'd have to set subwoofer=Yes but now things get tricky. If you set the fronts to Small (xover set), all the bass below the xover setting in the receiver, including LFE, will go to those external subs (LFE always goes to the sub). Not much content below 80 Hz will reach the front speakers so the internal sub will do pretty much nothing.
If you set the fronts to full range (Large), the internal sub will handle anything ~80 Hz because the speaker is getting the full range and the internal sub is taking the 80 Hz and below for itself instead of the other drivers. But, bass for the other channels, including LFE, will go the external subs.
So it's a matter of trying to wrap your head around what kind of signal routing is going on with different configurations. Speakers with a built-in sub are generally meant to be used stand-alone without additional subs. You can use additional subs if you want but like I said it gets tricky.
