The ALO Rx turns out to have a gain switch hidden behind the front face plate. All you have to do is get a suitable hex key and open the front faceplate. Then the gain switch can be found under the PCB, below the power switch. In high gain, the Rx was able to drive the HD800 to ear deafening volume levels. I didn’t find out how loud it can go, because the set up has a potential to do some permanent hearing damage.
Playing some Black Eyed Peas, the skin below my ears started vibrating just from the amount of bass coming out the ALO Rx and HD800 set up!
update: Ken from ALO wrote:
Because the default setting should be sufficient to drive any headphone we do not recommend that the amp be switched to hi gain setting. Also caution should be taken with removing and replacing the screws because the aluminum panels are rather soft so continuous removing and replacing the screws will lead to a stripped enclosure. Moreover over tightening the screws will strip the screw.