Reddit reviews AUDEW Car Audio Amplifier Volume Control Knob Bass Remote RCA Level LC-1 Universal
We found 2 Reddit comments about AUDEW Car Audio Amplifier Volume Control Knob Bass Remote RCA Level LC-1 Universal. Here are the top ones, ranked by their Reddit score.
Although not nearly as nice (but much cheaper) I believe this would also get the job done.
It would be helpful if you could provide links to the manuals of the products you want to connect, or at least the make/models.
A speaker out to line level adapter is probably not the correct solution because the settings on that adapter would need to change with the volume of the amplifier you're feeding it with.
Every commercial receiver, pre-amplifier, or integrated amplifier I've ever seen has at least one of the following line-level(-ish) outputs: headphone jack (TRS), tape out (RCA), pre-out (RCA), equalizer out, TV (RCA, HDMI), or SPDIF/Toslink (digital).
If you absolutely want to connect using a converter instead of connecting the sub properly, consider volume control devices:
https://www.amazon.ca/AUDEW-Amplifier-Control-Remote-Universal/dp/B014MCLUS2/
https://www.amazon.ca/XCSOURCE-Universal-Amplifier-Controller-MA713/dp/B01IR13NGG/
(I can't speak to the quality of any of those, the entire product category is fancy boxed variable resistors of some kind, so around $1-2 in parts costs.)