This guy gets it! 
I’ve been doing similar experiments using my venerable HD580s. As a test I bought a Fiio BTR1K, thinking it wouldn’t be enough to power the 580s, but would be enough to see if it was worthwhile continuing the experiment with the BTR5 or similar.
To my surprise, the BTR1 has enough gain for the volumes I listen at (not that high) and the music (mostly jazz and classical).
It can just be clipped to your clothing as mentioned above, but it also slots perfectly onto the adjustment part of the headband. The long 3m stock cable is a hassle, but I’ve ordered a shorter 1m one.
Whilst a fully custom cable would best, my soldering skills are akin to a drunk monkey. So if that shorter one works out I plan to cut it down to just above the Y connection and use a 3.5mm terminal block (ie solderless) connector .
It is also possible to buy universal Bluetooth cables with MMCX connections (Shure and Westone), Sennheiser connections, and custom ones (Plussound).
Their usefulness will depend on whether you feel the tiny amps can drive the heads properly.
I’m probably going to try out the Sennheiser ones.
I would strongly suggest trying the up4 over the btr5 . It’s has more single ended power and is warmer sounding.
I have a pair of Audeze iSine10s which are such a surprisingly great performer. I use them wired with my iphone via cipher cable or 3.5mm jack into my Oppo HA2 SE DAC at my desk.
I have been thinking about buying the Audeze cipher Bluetooth module https://www.audeze.com/collections/accessories/products/audeze-cipher-bluetooth-module but after some research I think the BTR5 beats it for features and can be used with other headphones etc. Think this is a better option or am I overlooking something?
Would say the BTR5 or Qudelix 5K both fits the bill as alternatives to the Audeze Cipher BT cable.
Edit: Or somthing like the Ikko Arc BT neckband maybe.
@C4PPY thanks for the response.
I pulled the trigger on the BTR5 a few months back and I found that it pairs really well with the iSine 10. For what it worth it drives my BLON03s very nicely as well.
cheers
Copied the idea in this thread and used a BTR5 to turn my PC350 into low latency wireless cans. Thanks to OP for the idea. Somehow we both ended up using the same yellow aux cable too, though I don’t recall where mine came from
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Has anyone created a step by step with newer tech for the BT? I’d be will to buy a second pair of HD600’s for this project but I don’t know what modern parts to buy for the BT.