The x.1 Low Frequency Effect channel is not defined as a full range channel. In most cases its sampling frequency is scaled down to 240 Hz. So you have to insert a LFE filter in this channel to prevent alias frequencies.

The Starplugs LFE – Filter represents a single channel filter that can be easily inserted in your 5.1 Master section. Its filter shape is over 48dB/oct, so you can use cutoff frequencies over 90 Hz (standard) and get much more punch and sound than other solutions can give.