it's the cut-off for the L/R as set sa receiver. if you have say, an AVIA DVD, you can do a frequency sweep of your system to check on how low can your speakers go. flexible naman ang choices natin as we have 40, 60, 80, 100, 120, at 200Hz to choose from. then you can set it up differently for each of your Fr/C/R speakers. yung SUB naka-direct in (LFE input) lang para iwas sa double cross-over. try mo muna single input, usually sa Left channel. if that proves too soft you can try connecting both using a y-splitter.