Better USB cables, e.g. the Audioquest Diamond and Cofffe make a considerable difference. When I was first started using digital devices, I used some good, but not great USB cables from Kimber (Silver) and Audioquest (Carbon). Originally, I didn't think, that with digital signals, a higher specification USB or SPDIF cable should make a difference, but much to my surprise, in listening tests with four individuals present, they did. Virtually every person agreed on the improvements and the degree of improvement. I eventually bought used Audioquest Diamaond and Coffee USB cables for my high-resolution Conrad-Johnson loudspeaker system and they improved the sound quailty from my digital source considerably. As did the Shunyata 75 ohm digital coax RCA SPDIF cable for my Eitr. Its not sublte; anyone can hear the improvements these cables bring. As for what people should spend their money on, its for each person to decide what creates a value proposition for them when it comes to audio gear. If you were to ask Wasupdog, I would bet you breakfast that he's not giving up his Audioquest Coffee USB cable. For me, the Audioquest Diamond/Coffee cables made such a significant improvement, that buying them used for 50% off retail was a value propostion for me.
Oh, and BTW, what makes even more difference are aftermarket power cables e.g. Shunyata.