Originally Posted by cjv123
I'd stupidly thought there was only 1 crossover, on the AVR, that determined which frequencies were sent to the sub as opposed to the fronts. You suggest there are two.
I think what you first asked about was the crossover setting and only the crossover setting. Whether your receiver has a separate LPF of LFE setting, I do not know. But if it does, the LPF of LFE (low-pass of LFE) setting determines the upper ceiling of the LFE channel's frequencies. The LFE channel can contain info as high as 120Hz. Under normal circumstances, you probably want to set this to 120Hz so that all of the LFE channel is being reproduced by the sub. There are certain instances where this could cause or exacerbate some localization issues. Like when the subwoofer is located to the rear. In these cases, a lower LPF of LFE setting can be used to alleviate the localization.
The LPF of LFE setting is not affected by the speaker channel crossover settings and vice versa. So, you can crossover your speakers at, for example 80Hz, with only sub-80Hz frequencies being sent to the sub from any crossed-over speaker channels, yet still allow the LFE channel to get through to the sub all the way up to 120Hz.
In the not too distant past, when most AVRs only allowed a single global crossover setting to be applied to all the speaker channels, and there was no separate LPF of LFE setting, the LFE channel was actually truncated at that single global crossover setting. So, if you used an 80Hz crossover, the LFE channel was truncated at 80Hz. Nowadays, with that available LPF of LFE setting, the full LFE channel, all the way up to 120Hz, can be sent to the subwoofer even though a separate and lower than 120Hz crossover setting can be used for the speaker channels.