On my Yamaha speakers the right speaker has multiple outputs. One goes to the left speaker, and two are left available for sub woofers or more speakers.You can plug the sub woofer directly into one output from the right speaker (the "right" one on my setup is the one with a volume knob
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Can you do this with the HK's and dispense with the splitter? I think what you're trying to do is making the speakers run in series, which increases impedance (resistance) and will cut your volume by around half.