Never had the chance to hear the new grace, but its predecessor was incredible. Gobs of detail, clearly a world class unit. The usb pre seems like the wrong tool in the case, its a mic pre not a playback DAC, and the Benchmark is a very nice unit, but is second class to the grace.....but then the price is too.
Get the Grace and forgetaboutit. Its also a world class HP amp as well.
it's also twice the price of a usbpre2, which is designed as a in/out sound card first, and field preamp second (look at how standalone mode works as an example, it's ergonomics as a preamp are another one).
I use mine that way a good 98% of the time. The headphone out
is crap if you have low impedance headphones (32ohm and under) which have a high sensitivity, but otherwise it's fine. The Grace's headphone amp stage trumps any of the other units listed, period. I had a 901 for a while and liked the amp section, the DAC was trumped by the usbpre2s, but I've heard it improved in the 902/903 units and is definitely up to snuff.
At the end of the day (IMHO), it's Grace > Sound Devices > Benchmark. I do agree that the Benchmark is behind the newer Grace Design models, I wasn't as enthused by it at a listening meet up as I was expecting, maybe it's because I was thinking about the retail price when I was listening to it. ymmv.
The real question is, do you want to pay for the Grace for what I see as a small quality increase from the usbpre2 (assuming you skip the headphone outs)? If you do use the headphone outs, it's a much bigger difference and I would definitely recommend the 902/903 units, but if not, it becomes very hard for me to justify it. From a playback perspective I haven't heard anyone over at head-fi rave about the Grace DAC stages in later products compared to stuff like the Lavry DA10/11 which is approximately the same cost.
And yes, USB mode on any of these will be trash with a mac. I refuse to use usb audio for the exact reason you state, it's a mac problem and not a unit issue. stupid sleep mode...