Differences between R-27RE and R-27HE:
The R-27HE has larger dimensions and a heavier chassis, a more complex regenerative power supply design, and higher output power (15W at 25ohm).
To reduce costs, the R-27RE applied a smaller and lighter chassis, redesigned the power supply board that keeps the sound quality at maximum and reduced the output power (10W at 25ohm), which is still enough for most headphones.
The R27RE is the more affordable choice.
The R-27HE boasts the highest performance.