Yeah, the length of cable might be attenuating the signal.
If you don't have the owner's manual for the sub, you can get it
here. Thanks for the other part number, btw. That was the ticket to finding the manual.
The manual states that if your receiver only has one subwoofer output, then you should connect to the "L" line-level input on the sub. In other words, you don't need the y-splitter cable. That will help with the signal strength.
Let me look some more into the manual, and I'd say remove the y-splitter to check it out in the meantime. I'll also look into increasing the subwoofer output level on the receiver if you have questions about that. You can do that to compensate for at least some of the loss in the cables.