Reddit mentions of JSAUX RCA Cable, [6.6ft/2M, Dual Shielded Gold-Plated] 3.5mm Male to 2RCA Male Stereo Audio Adapter Coaxial Cable Nylon Braided AUX RCA Y Cord for Smartphones, MP3, Tablets, Speakers, HDTV [Grey]

You are looking for something along these lines:

https://amazon.com/Converter-192kHz-Techole-Aluminum-Converter-Headphone/dp/B07MPF4F68/ref=sr_1_4?crid=27BU29OQEP1Y&keywords=toslink+to+rca+converter&qid=1563574281&s=electronics&sprefix=tosl%2Celectronics%2C160&sr=1-4

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https://amazon.com/ROOFULL-Digital-Converter-Toslink-Optical/dp/B07142SV1D/ref=sr_1_3?crid=27BU29OQEP1Y&keywords=toslink+to+rca+converter&qid=1563574281&s=electronics&sprefix=tosl%2Celectronics%2C160&sr=1-3

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Or, if you are insane & wealthy:

https://www.berkeleyaudiodesign.com/alpha-dac-reference-series-3

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p.s. I believe you will need something like this if you don't have it already:

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https://amazon.com/JSAUX-Shielded-Gold-Plated-Compatible-Smartphones/dp/B07D8M5DML/ref=sr_1_2_sspa?keywords=RCA+male+too+3.5&qid=1563574516&s=electronics&sr=1-2-spons&psc=1

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https://amazon.com/AmazonBasics-3-5mm-2-Male-Adapter-Cable/dp/B01D5H8JW0/ref=sr_1_1_sspa?keywords=RCA+male+too+3.5&qid=1563574516&s=electronics&sr=1-1-spons&psc=1

Basically, you'll need a box to convert your S/PDIF output to analog for your Logitech to take in.

Something like this should do the trick. NOTE: I haven't used this product, and can't make any guarantee it'll solve your problem, but it claims to do the job you're looking for it to do.

Then, you'll take some dual RCA to 1/8" cables, plug them into the correct outputs on said box, and into the correct inputs on your subwoofer.

You'll also need an S/PDIF cable to go between your xbox/laptop and the above box.

Another option may be this. It'd act as an external sound card for your laptop as you suggested. It also has an S/PDIF input, which your XBox could theoretically plug into... I'm just trying to find the manual to confirm that this input could be passed to the outputs lag free...

Edit: from the manual, "The signal only passes through to the S/PDIF OUT.

The S/PDIF optical pass-through port on the Audio Adapter supports two-channel audio, but does not support 5.1 or 7.1 audio." So looks like no, the StarTech link I shared won't work.

The MICCA G2 has a 3.5mm STEREO output, that also serves as a 3.5mm Optical Output.

You simply need a 3.5mm Stereo to 2x (Red/White) RCA. These are fairly common.

https://www.amazon.com/s/ref=nb_sb_noss_2?url=search-alias%3Daps&field-keywords=3.5mm+Stereo+to+RCA

However, most of these cables assume the RCA connector will be close together, so you want one where the place where the cable joins 2x into 1x is wide enough or can be split apart sufficiently to each your studio monitors.

For Example, look at this cable -

https://www.amazon.com/JSAUX-Shielded-Gold-Plated-Compatible-Smartphones/dp/B07D8M5DML/

On the right side are a column of Photos, choose the second photo down. Notice that you can not move these RCA connector very far from each other.

However a cable like this Monoprice 3.5mm to RCA is two cables all the way back to the 3.5mm Plug, and the two cables can be split apart as far as you need them. This come in 1.5ft, 3ft, 6ft, 10ft, 15ft, 25ft, and 35ft so it can accommodate whatever length you need.

https://www.amazon.com/Monoprice-105597-3-Feet-Premium-Stereo/dp/B0094A1F3S/ref=sr_1_15?ie=UTF8&qid=1538364593&sr=8-15&keywords=3.5mm+Stereo+to+RCA

So, the 3.5mm out on the Micca G2 is STEREO. Use a 3.5mm Stereo plug and you can run each channel out to the appropriate speaker.