Sub wiring help, high level speaker terminal to RCA line level Sub inputs

I’ve done some more reading and found using an XLR to RCA cable is a bad idea. Mainly because to make the cable the XLR (-) lead is shorted to the ground lead on the RCA side, which could cause feedback issues and potential damage, so those options are out.
https://forum.audiogon.com/discussions/cd-player-xlr-output-buzzing

So my options are then what yogiboy and jtcf suggested to use an RCA Y-splitter, or my original though of using a speaker terminal to RCA cable.

rhern213, sorry for the bad advice above. I've actually done this. The simplest solution is a Line Out Converter or LOC 
https://www.parts-express.com/stinger-sgn111-30-watt-fixed-line-out-converter--268-482

This is simply a couple of resistors between the wire inputs and RCA outputs. One resistor drops the amplifiers output voltage down to line level, the other provides the impedance load to whatever the output component needs- the sub in your case.

My integrated amp didn't have a pre-out and so when my new sub amp had like yours only line-level inputs I researched this to death before modding my amp. In my case I am comfortable modding and installed the parts inside my amp. It does look better that way. Electrically however it is identical to the above LOC, which I'm sure you will find a whole lot easier to use.

Why not send it back for a replacement with Rythmik subs that comes with high level speaker input. I see no reason to apply ‘band-aid’ solutions being suggested by Rythmik fanboys.

Or you can excercise Option 4. 

IMO, using high level input connectors offers the best possible integration with your mains speakers.

“The high-level input is designed to accept the stereo (two-channel) signals from the speaker terminals of your receiver, integrated amplifier, and basic amplifiers. This has the advantage of ensuring that your subwoofer receives exactly the same signal as the main speakers, which means that the character of the bass from the main system is carried forward into the Sub-Bass System”.

IMO, using high level input connectors offers the best possible integration with your mains speakers.

Okay so here's the thing. If you look at the schematic for subs with high level input connectors, the circuit is exactly the same as the LOC described above. Exact. Same.Thing.

The only difference is the circuit is inside the sub, as opposed to being in the LOC. But wherever it is located, it is the exact same thing.

Millercarbon’s suggestion looks to me like an excellent one in this situation. The 30 watt specified power handling capability of the speaker-level to line-level converter he suggested seems reasonable in relation to the power capability of the OP’s amp (20 watts into 8 ohms; 40 watts into 4 ohms), considering that the amp is only likely to be run near max power when the music reaches very brief dynamic peaks. Also, problems can arise when driving a powered sub via the outputs of an amp that is balanced or bridged, depending on the internal grounding configuration of the sub and the amp and on whether one sub or two is being used. But in this case the OP’s amp is neither balanced or bridged, when it is used in stereo mode.

OP, glad you found the thread you linked to, which I remember well as I was instrumental in diagnosing the problem. As you may have seen in the thread, an XLR-to-RCA cable or adapter that is specially configured with XLR pin 3 unconnected can be a suitable solution in many such cases. But many designs which provide both XLR and RCA outputs provide the same signal to the non-inverted signal pin of the XLR connector that is provided to the RCA connector, in effect splitting that signal internally. Meaning that the results of using the XLR connector in conjunction with a suitably designed adapter, while using the RCA connector to drive the other component, would likely be little or no different than using the Audioquest splitters others have suggested. (I’ve used those splitters myself, btw, in a different application, with fine results given that the output impedance of the component providing the signal was low enough to properly drive the two input impedances and the capacitances of the two cables that were involved).

In any event, though, it appears that Miller’s suggestion is the preferable one in this case, for the reason Lalit stated. The only possible issue I can envision is the unlikely possibility of a ground loop-related hum.

Regards,
-- Al