My apologies, I completely misunderstood what you wre asking. I was thinking frequency, not voltage levels. I believe I have seen speaker level to line level adapters at Parts Express and Monoprice but have not looked recently. All you need is a resistive voltage divider plus blocking caps to keep any d.c. out of the sub amp (use on + and - sides to isolate ground in case the receiver has a bridged output stage). You can also get in-line attenuators, but they do not always provide d.c. blocking and I would want that in this case. YMMV.
I did not see frequency specs on Amazon and did not try to track down the manufacturer's specs. Most transformer-based designs do not go below 20 Hz if that so I would double-check before using in a subwoofer application. If you can DIY, resistors and caps should do the trick.
100 W into 8 ohms is 80 Vpp (28.28 Vrms). Your amp probably expects around 1 Vrms so you'd want about 15 dB of attenuation.
HTH - Don
"After silence, that which best expresses the inexpressible, is music" - Aldous Huxley