I'll take a stab at this, but Brian's the expert. I am a hairy-knuckled design engineer and my day job involves much higher frequencies than subs put out...
@ vantagesc: Figure 3.3 on page 35 of the paper shows the LFE signal fed into the main L/R outputs. As for compression, the LFE signal is recorded at a lower level than the rest of the channels, then boosted at the end into the sub. There's more to it than this (and I am still reading about LFE in general) but this was done to prevent
excessive compression that might otherwise occur with a very high-level bass signal. That is, a large bass signal could "use up all the bits" so there aren't any (or few) left for the important upper frequencies riding on the large bass signal. The LFE spec was designed to prevent that, and thus the various gain blocks sprinkled around the diagrams.
If you are talking about routing low-frequency content to your mains and they (L/R main amp and speakers) don't have the dynamic range, then yeah you could get compression, but that's a problem anytime you head for 11 on the gain knob...
@sdurani: As stated, you are exactly right and I agree. However, the dozen or so AVRs I was able to find out about (reading the literature and/or measuring) in the $1k'ish range and below tend to use the first diagram for "small" fronts and second for "large". They set the HPF at 80 Hz for the "upper" speakers and LPF at 80 Hz to the sub automatically for you ("it's a feature" -- especially with the lower-end receivers). Thus, since the LFE signal is sent only to the sub if you select "small" mains, LFE content between 80 and 120 Hz is lost. (Not completely because the filters have finite roll-off but you get the idea.) Some, but not all, of the AVRs had the ability to tweak the subwoofer setting independently of the mains, and the higher-end AVRs offer much more flexibility in setting both points. So, if you have the control you can correct the issue just as you say, assuming your sub has high enough bandwidth (higher than I care for, but that's a personal thing). Of course, that same control means you could lose information but setting them wrong... Disclaimer: Some of the AVRs I checked are older ones I am using at home, so it could well be all the newer ones allow you to set the sub's LPF to 120 Hz. I have noticed some do.
I much prefer full-range L/R speakers but am also much more music- than video-oriented (so far, anyway). As for my kids, well, the opposite, natch!
Again, Brian or someone with more experience/knowledge will hopefully step in and correct my mistakes. - Don