Interconnect or USB cable

Unfortunately, it just depends on your components and system. I have had analog, digital, USB, or power cord make a huge difference… or not. I typically I do speaker cable first, then interconnects, amp power cords next, and finally USB, My dealer /friend says the biggest difference he has heard was with a USB. But for me it was a digital coax first, interconnects, amp power cord. But it just depends.

What equipment do you have?

Help others help you, list your equipment and current ICs and USB. 

great point audphile1, thanks. here's my system:

Primare i30 integrated amp, Ascend Acoustics Sierra 2EX speakers, Rythmik L12 subwoofer, Triode Labs American speaker cables, Audio Envy Studio Prestige Interconnects and Power cords, Acoustic BBQ Full Rack USB cable, Schiit Bifrost 2 DAC

Depending on how much you were planning to spend….

The best USB cable I have tried is the Audience AU24SE+. 
What to upgrade first? Tough to say…if you get better ICs you can only realize as much as your DAC and USB cable can dish out, and other way around if you upgrade USB your ICs may become a bottleneck. 
 

In my opinion, best way to approach it at this point in your system is to upgrade the DAC. Forget cables for now. But again, just my opinion. 

Me’all?  Neither. I’d get another sub first. I think that will give you the most significant improvement.