With the soundcard having only analog outs, you're forced to do bass management in the PC, so the LFE and other bass is mixed before you can get access to it. This is based on that fact that most AVRs with 7.1 analog inputs do not have A/Ds to convert it to digital. If you had such an AVR, then the player would not do the bass management, so the LFE is available. Could run it to a Y splitter and feed it to the shakers. But then it is at a fixed level, as there's no volume control at the player's output.
Even if you has an HDMI soundcard and AVR, I have not seen any ability to have both LFE and summed bass on separate outputs. It's one or the other.
Now if you could accept sending the same subwoofer signal to the shakers, it's easy.
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