the only stupid question is the one not asked
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I would be cautious choosing any of the devices that have been suggested, including the devices shown as alternatives on the Amazon page that was linked.
Many of them indicate maximum power ratings in the area of 30 to 80 watts, which may or may not be adequate for your purposes. Some, such as the Pyramid NS60, have low input impedances (10 ohms for that model). That will consume a significant fraction of the power capability of your amp; it will significantly lower the impedance the amp has to drive; and it will generate heat. Most of them do not specify output impedance, but if the input impedance of your sub is as low as say 5K or 10K, as is often the case, I wouldn't be surprised if the output impedance of some of the adapters is too high for good sonics.
None of the adapters I have seen provide specs that are both complete and clearly acceptable.
Personally, I would build something myself. Aside from the connectors and a small enclosure, all that is required are two resistors on each channel. Let us know if you'd like further specifics on that.
On the DIY issue, I've used resisters myself in a preamp out, that needed a subwoofer out also, but never tried speaker out. This was done on disposable gear also. Some of these adapters use what appear to be isolation transformers. Others have just resisters I think. I've never tried one for home audio, because I don't feel safe not knowing what they may have going on inside them. Link for a transformer one (below) that I don't think I'd use either, but at least may provide isolation, if needed. But it may also short, or have its own impedance characteristics. For this reason (to many unknown variables), I never no of any to recommend. [http://www.sonicelectronix.com/item_18735_Absolute-HLA600.html]