Originally Posted by Rolls-Royce
I may understand better than you think I do. It's a low pass filter, not the sub's crossover, and it only impacts info sent in the LFE channel. It likely wouldn't cause problems with a sub crossover set at 120 Hz. That was just a WAG on my part. But crossovers are also filters, and overlapping them could cause unexpected effects. However, any time you increase the upper frequencies a sub is asked to put out (and that is part of the stated point of the OP, "to get max info from the sub", is it not?), you will pay a price with some loss of lower end extension. Agreed, this would occur only if a signal contains an LFE component, and a sub with very high power and superior design will suffer less from it. But it's precisely the time when you are trying to improve the sound that you could be limiting its performance somewhat at the lowest frequencies.
Without wishing to start a debate on the current merits of the THX program, and looking back at their original stated purpose of improving theater (commercial and later, home) sound, if setting this control to 120 Hz is the best choice, why haven't they or the CE manufacturers made it the default? Or better yet, just set it there without giving us an option to change it? Even the Audyssey FAQ kinglerxt referred to in an earlier post said that one probably wouldn't notice a significant difference when changing the LPF of LFE from its 80 Hz default. So why bother?
I wrote the Audyssey FAQ for this thread, so I'll chime in here. The THX conceit of 80Hz is primarily for systems designed to fit THX specs, i.e. subwoofers designed specifically for sub-80Hz output. THX-certified receivers with a LPF of LFE option use this as the default. However, the LFE channel is spec'd by Dolby Labs for output up to 120Hz. Now, I do realize that we've said in the FAQ that the difference in this setting will be minimal, but IT IS THERE. You have to remember that with any sound, you'll also have harmonics above that sound that give it its perceived tone. That's true even of deep bass.
A good example is the opening of Lord Of The Rings: The Fellowship Of The Ring. When Sauron dies, there's a bass sweep that occurs that is balanced between the main channels (for the directional part of the sweep) and the LFE channel. With critical listening, you can hear a difference between this scene with the default 80Hz THX LPF and the 120Hz that Dolby specs dictate for the channel (i.e. the LFE channel is brickwall filtered at 120Hz by its very nature). Now, audio engineers/mixers don't tend to place much above 80Hz in the LFE channel, so whether this will make a difference will depend on each individual movie or source. That said, the goal of good home theater should be ACCURACY, specifically the accurate reproduction of what the audio mixer intended you to hear. And the only accurate way to reproduce the LFE channel is to set the LPF of LFE to 120Hz so you aren't re-filtering a channel that has already been filtered at 120Hz.
As for this making the subwoofer localizable, it won't. As I said, very little above 80Hz is placed in the LFE channel. The harmonics that will exist above 80Hz, however, are important to giving the sub-80Hz material an actual tone instead of just a generalized rumble. And since the majority of significant energy is still below 80Hz, you don't have to worry about localizing the sub the way you do when redirecting bass from the other channels at too high a crossover point. For this same reason, you're not going to eke out any more noticeable performance from your subwoofer by filtering just the LFE channel at 80Hz, because the harmonics above 80Hz in the LFE channel are so minimal (though they are important).
As for what you said about overlapping crossovers, there's no effect whatsoever if you're doing things right. Older receivers without independently adjustable crossovers for each channel would often sum all bass with the LFE channel and then place a filter on the summed data for output. This worked fine in concept, since it usually did so at 80Hz (especially with receivers that only had the LARGE/SMALL option), but it truncated the LFE channel in the process. AVRs with independent per-channel crossovers do the filtering BEFORE the sound is summed for the subwoofer output (using digital filters that mimic Linkwitz-Riley filter behavior - flat amplitude response across the crossover range with no phase shift), so you don't run into any filtering issues. For instance, you can have 60Hz on your mains, 70Hz for your center, 80Hz for your surrounds and 120Hz on the LFE channel (which is actually how my system is set up), and no worries. The only thing you have to ensure is that the crossover on the subwoofer itself doesn't cascade with the overall subwoofer output and create a gap in frequency response (e.g. all channels set to 80Hz on the AVR with an 80Hz crossover on the subwoofer will reduce output at the crossover point by 6dB, since the low pass gain on a L-R filter should be -6dB at that point). So long as the crossover/filter on your subwoofer is set well above 120Hz (at least 140Hz preferably, since the crossover has a gradual slope), you're accurately reproducing all audio that is being passed to the subwoofer.
Ideally, you should disable your subwoofer's crossover altogether or turn it to its maximum setting because all filtering is already being done at a digital level in the receiver before output to the subwoofer. Active crossovers can cause delay in the signal chain, which can affect the proper distance/delay setting to align your subwoofer's phase with the other channels (since distance/delay essentially acts as a variable phase control by its nature). The Audyssey detection makes this a non-issue, however, since it calibrates subwoofer distance/delay based on acoustic distance, which takes into account any delays induced in the signal chain.
Having said all that (and I know I just typed a freakin' book), it's your system. Set it the way you want. But the information presented here is to ensure accurate reproduction of the intended sound, and if that's what you're aiming for, I hope we've helped.