DAC and groundloop issue

I'm having a ground loop buzz noise when I connect my benchmark DAC---connected via digital cable from CD to DAC-then DAC to pre-amp. The amp is a rogue stereo 90 tubed unit-----doesnt seem to occur when i substitute a solid state amp in the system-
Go to the Jensen Transformers webside. They deal specifically with ground loop issues. You would put a transformer between the Benchmark and Rogue. Their products are high quality, inexpensive, and work well.