For reasons connected with this particular Pi having another job to do as well (and a hat to help with that) I need to use a USB DAC. I also need the Pi to be in one place, and the amp in another, about 2.5m away. Cables between the two will have to pass near to ethernet cables and power cables, monitors and speakers, and potentially a wifi access point (yeah, it’s a crowded office).
Would I be better off placing the DAC near the Pi, with a short USB cable and a long audio cable. Or placing the DAC near the amp, with long USB run, and a short audio cable? The audio will be 3.5mm to RCA, because the DAC I have is a Cyrus Soundkey.
The USB cable should be the least susceptible to interference since it is only passing digital data which is retimed in the USB DAC. If the data is corrupted it will be very obvious - not at at all subtle, but I don’t think you will get any issues.
Try not to run the audio lead parallel to the power cable since low level mains hum may be picked up. OK to cross the power cable at right angles since this will give very little pick up.
More expensive cables do not give better sounding results than reasonably priced cables! Just buy cables with good solid plugs and neat construction. Amazon Basics cables are generally good value. There is a number of you tubers out there who will try to tell you otherwise but they do not support their assertions with any measurements or proper scientific analysis.