Although you asked about sonics rather than functionality, I’ll mention that if the UR S6 produces too much volume at low settings of the volume control I suspect that the Manley will also.
The unbalanced outputs of your DAC have a "full scale" (maximum) output level of a bit more than 3 volts according to Stereophile’s measurements, which is quite high, and the Manley will be driven to full power by an input of approximately 0.2 volts with the volume control at max. Which suggests that you would be using the volume control quite low within its range.
The Leben, on the other hand, is spec’d such that 0.6 volts is necessary to drive it to full power with the volume control at max, while at the same time full power is somewhat less than the Manley can provide.
You might consider addressing this issue with a pair of Rothwell in-line attenuators at the inputs of the amp. Some users (including me) have had positive experiences with them, while others have reported that they compromise dynamics to some degree. But I suspect that the relatively high input impedances of these amps would make a positive outcome with them more likely than not.
Good luck. Regards,