Long unbalanced RCA or long USB cable?

"IF", your using the USB cable that lets, "Makes", the signal bypass all, "Sound Enhancement" software and second party software? Then that should be all you need.  "It's the USB cable with the (Square-ish) USB connector on one end ". 

What type of "Line Source" do you have? Mine sounds insane! No one ever believes me when I explain what it will do. Until they hear it..... Ha ha.Then they always get that "Confused" look on their mugs!

USB cables either work, or they don’t. Analog cables, like unbalanced RCA, add more resistive and reactive loads as they get longer, thus slowly degrading the signal as length increases. Use your 15’ USB cable. 

It'll be the debate of "it's only '0's and '1's"
Although USB has a very reliable CRC check, USB audio devices use isochronous communication which has no hardware error-check. Which means that the device itself has to be able to recover from occasional errors.
If you don't have the crappiest USB cable (no need for overpriced cables) and don't go over 5m it shouldn't be a problem (if there is you'll clearly hear dropouts).
You could also use shorter cables with a USB hub in between (you can only chain 5 hubs).

They either work or they don't?....
 A year ago I was playing with a new DAC/Streamer I purchased. For some reason it didn't like my wifi. That companies "Support" explained to me then that the "USB" might be a better option for me. AND they also explained that using an "USB" with that squarish connector and your laptop/PC, was the only way to get "Just the signal", With no data compression, filtering or sound enhancement program changing the audio and etc. "That squarish" connector is read by your laptop and once found thinks it is a "Printer" I think. Or something like that. And it clears a path for just the audio file in it's entirety.  Thus bypassing all the crap.
    It is a very reputable company "By the way", as well. And I used that set-up for a time in my shop.