Hi Joey,
No, there's no reason to use a Y cable to hook up your EP500 to the mono LFE feed from your AV receiver. Virtually all subwoofers will sum the L and R low-level inputs anyway. Traditionally, if a subwoofer has separate Left and Right channel line-level (low-level) inputs, the Left input is designated as the single connection to use.
Using a Y-cable will not increase the signal level to the subwoofer, despite the curious advice from the B&W manual.
If you check your EP500's rear panel, there is one RCA line-level input jack and a second RCA line-level output jack. There are also duplicate XLR balanced input and output connectors.
If you operate a second subwoofer, use the same type of line level output--RCA or XLR--as you use for the input.
Regards,
