Not sure if you could port the signal through your xbox cables.
Is the subwoofer powered, does it plug into it’s own outlet? If not then connecting it through your receiver isn’t going to work. Sounds like your running HDMI to your receiver. Not sure how or if you even can route a separate analogue signal back from your Xbox to the Buttkicker amp. You might want to search or post at the AVS forum since there’s lots more people there and more likely someone might have dealt with this:
http://www.avsforum.com/avs-vb/forumdisplay.php?f=113
What sort of subwoofer output connection does you receiver have?
The Y-output is just an inexpensive splitter with a male RCA adapater on one end that split’s the signal to 2 female RCA adapters one of which then goes to your subwoofer and the other to the Buttkicker amp. Here’s and example:
http://www.cablesnmor.com/rca-adapter-cable.html
If your subwoofer connects using double stranded speaker wire then you need a “speaker level interface adapter.” If you don’t have the one that came with the kit then call the Guitammer company (maker of Buttkicker) and see if you can order one:
http://www.thebuttkicker.com/contact_us/index.htm
If neither of these apply to you then you could try calling Panasonic to see if they have any ideas.