Congrats on your new DAC.
As fa as a headphone cable, Mogami W2697 from Performance Audio is a good choice @ 31 cents per foot. It has 3 conductors, a twisted pair wrapped with a shield but is very small diameter. I just made a cable for my DT-1990 Pro and it is excellent! (see photo) Really low capacitance. Completely busts the myth of needing thick wires for superior performance.
https://www.performanceaudio.com/mogami-w2697-miniature-balanced-mic-cable-by-the-foot.htmlYou could get 30 feet of W2697 for $9, and cut it in half for your 15 foot requirement.
Get a TRS plug for the source and two TS plugs (or other 2 conductor plug) for your headphones. HiFiman is like this:
https://www.performanceaudio.com/amphenol-km2pb-au-3-5mm-ts-mono-mini-plug-cable-connector-black-gold.htmlI didn't see you mention what headphones you are using.
Strip the source end of both cables and tie the all white wires and shields together and solder it to the sleeve on the TRS plug.
Solder one red wire to the ring of the TRS and the other red wire to the tip. Put a mesh jacket (have to figure out jacket size) around both cables up to the point where they split to the drivers and then heat shrink at the split point.
This 1/4 inch Techflex works great, fits 2 cables of W2697 nicely:
https://www.performanceaudio.com/catalogsearch/result/?q=Techflex+Flexo+PET+Expandable+Braided+Sleeving+%281%2F4%22+Black%2C+By+the+Foot%29Strip the ends of the of the other 2 cables and cut the shields away as they will not be attached. Solder the red wires to the tips and the white wires to the sleeves of the TS plugs. Use red or black heat shrink at the TS plug on each cable to distinguish right from left. Check with meter unless you know which side is which.
What's left is a 15 foot premium cable with good shielding for less than 20 bucks. IMO, the DIY route is a great way to go for cables if you have the time to do it.
The DT-1990 Pro required only a single cable of W2697 because the connector at the left driver is only 3 pin. The 3 conductor wire is perfect for this. I have a plan to change that connector to a 4 pin XLR and wire it correctly for balanced operation.