Except for a difference in gain applied to the input channel, you should hear no difference whatsoever between "variable" or "calibrated".
What happens when 196/24 files are played via the DAC1 USB?
How can I use two Benchmark USB DAC units at once?
In some cases, upsampling will improve the output of your D/A converter. The low-pass filters incorporated into the upsampling process will essentially replace the filters in your D/A converter. If the upsampling software has better filters than those built into the D/A, then you may see an improvement.
Caution: Do not remove the internal safety cover that encloses the AC power entry.
Note: The cover is designed to fit tightly against the side of the unit.