How to defeat dynamic USB port assignment

It's been suggested that a DAC sounds better when it is the only device on a USB port. I have reconnected my USB 2 DAC to different ports on my Mac Mini Core2 Duo until it shows up as the only device on a Hi Speed Bus. Great! But each time I restart or turn the DAC off the assignment has changed--usually to a USB 1 port. Is there a way to defeat this automatic assignment?