The ADC1 converters have the ability to provide 16-bit outputs on the auxiliary output while providing high-resolution outputs on all other outputs. The 16-bit output mode is always TPDF dithered. The auxiliary output can be set for 16-bit operation or 24-bit operation. All other outputs are always 24-bit.
The ADC1 converters also have the ability to output up to 3 different sample rates simultaneously. The USB, Main, and Aux outputs are independent. The sample rate of the USB output is controlled by the computer.
Except for a difference in gain applied to the input channel, you should hear no difference whatsoever between "variable" or "calibrated".
Vinyl places many restrictions on the mix and mastering engineers. High-frequency content must be limited due to the mechanical limitations of cutters and playback cartridges. Low-frequency energy must also be limited for similar reasons. Stereo content must be limited and stereo separation of the system is also limited. Vinyl cannot be aggressively mastered for increased loudness, the way CDs can.