Benchmark has made changes/improvements to their DAC line over time.
The DAC1 USB you have uses an Analog Devices AD1853 dac chip like its predecessors, but upgrades the stereo-outs to LM4562/LME49860 opamps, which should sound better(read: smoother) when connected to another amp than the original DAC1 that many people heard back in the day.
The DAC1 PRE updated all the op-amps to LM4562/LME49860 opamps, so the headphone-out should sound better too.
The DAC2 line changed the converter chip to the ES9018, which adds DSD64 decoding capability, if that is relevant to the content you play.
The DAC3 line changed the converter chip to the ES9028PRO. I have no idea if that sounds any better, or what it would take to detect a difference.
IME, the hardware component factors that affect the sound of a DAC is the converter implementation, along with the output stage. For me, the DAC2 line was/is the sweet spot when it comes to price::features::performance for Benchmark.