Just reviewing data from the internet:
To minimize loss, aim for a voltage drop under 3% for sensitive equipment.
The voltage drop calculator, says that 10 gauge wire over 100 feet should only cause a drop of 1.6V (1.33%) at 16 amps. However, Google says that the drop would be 5.4V (4.5%) at 20 amps.
The same voltage drop calculator also says that 6 gauge wire over 100 feet should cause a drop of 2.37V (2.0%) at 60 amps. Here Google says that the drop would be 4.7V (3.9%) at 60 amps, which is what I would need for a sub panel.
So, perhaps just go with the easier option, and run 3 x 10 gauge wires.

