When you say compromise the sound of your whole system, are there other sources, possibly. If not, thats a different story. You say that you don't have a preamp, so if you are using it to drive your Rotels, I think it will sound pretty good; that's why they put the volume control on there in the first place. The only place it might mess you up is on future upgrades. Eventually, if you want better sound, a separate active preamp will probably be necessary. Keep in mind, though, you will need a very good (Usually expensive) preamp to equal or better the sound of the DAC direct. A cheap one won't do it. Also, at that point, you will definitely want to bypass the DAC's volume control to get the best sound if, and when, you do upgrade. I'm not that familiar Benchmark so you would have to find out the details on that.
All said; I do have experience with this type of thing, but with other brands. My best guess is that you will be very happy with the sound. It's a sensible upgrade and if it were me, I would do it.