I’d just like to stress the importance of a physical volume control. I’ve seen occasion when the computers have jumped from, say, 50% to a 100% output level, for no apparent reason. Perhaps a minor bug somewhere. It’s very rare that this happens, but it can happen with a software controlled volume control. It takes just one occurrence at the wrong time, to have disastrous consequences.
Imagine that it happens when a musician is making a recording with headphones on. This could easily cause severe hearing damage or, in the worst case, total hearing-loss (ruptured eardrums).
I think it should be a legal requite that all audio interfaces should have a non-software controlled maximum output level knob or pot. Unfortunately it isn’t, so it’s up to us studio owners to make sure that we don’t jeopardize our talents carers (and life). I, for one, would not live with the knowledge that my studio has caused the ending of someones musical career.
Even if your amp has a physical input level knob, it is important that you never trust your computers or audiointerfaces software controlled volume controls. If you set your software controlled volume control to 50% and then set your amps input control, it could still cause serious damage if the software controlled volume control were to jump to 100%!
The way I set it up is:
- I have a physical volume knob inserted between the audio interface and the power amp. (Most headphone (cue) amps has internal physical knobs.)
- I set all volume controls (monitor volume and cue levels in Cubases Control Room, the computer output volume and the MR816 encoder) to full on.
- I then play a loud piece of music and check the meters reaches max.
- I then turn up my physical volume until I reach the maximum level I want to allow (and I should never touch this again).
I can then back down the Control Room monitor level and cue levels to a proper working levels, safe in the knowledge that no spikes will exceed my maximum levels (and that the musicians leave the studio with their hearing intact).
Of cause setting up a proper gain-chain from input to output, before setting up a maximum allowed output level, is a good idea. But that’s a tutorial for another time. 
The reason I use a separate volume control is that my power amp lacks physical input controls. If your amp have them, you don’t need to add a physical knob. If you do need to add a knob, any old spare mixer you have lying around will do. As an alternative, something like this (http://www.smproaudio.com/index.php/en/products/monitor-controllers/nano-patch-plus) is a good option. It even has a mute button that can act a panic button, if needed.