If you are doing USB and only USB, then you might consider one of the Wavelength USB DACs or the forthcoming Ayre USB DAC.
Another approach would be to add an Empirical Audio Offramp (converts USB at up to 24/96 to SPDIF, AES/EBU and I2S), then you are free to get any DAC. I think the Bryston BDA-1 is absolutely killer at $2,000. It has USB, but only for 16/44. I have not heard the Benchmark in a long time, but the Bryston handily surpasses the Lavry DA-10, which many prefer to the Benchmark. Again, though, this is via SPDIF, not USB. [Disclaimer: I'm using Empirical Audio into Bryston.]