All these devices are designed to a standard.
So if you have the gain on max in your application and also in Windows mixer then even if the recorded source level has been set to the 'loudest' possible (it almost certainly has) a well designed DAC should be able to cope.
Same story with the input to your speakers. Max signal out from your DAC/mixer or whatever should play without distortion even with the gain set on unity. It probably wont though on pop or dance. Anyway you will not want it that loud. Even if you are not worried about hearing damage your neighbours probably are.
Which brings us to another issue. Powered near field monitors are generally designed pro or semi pro use. There is an individual gain control on the rear of each unit. This is obviously inconvenient and users are expected to control gain via a mixer or, more usually these days, a suitable audio interface. You can still use a mouse and windows if you like but it's fiddly and slow.
So before you buy a basic consumer hi-fi grade DAC like the Modi take a look at a proper pro or prosumer audio interface. This will almost certainly come with separate gain controls for line and headphone output which will be conveniently situated close by on your desktop.
For little more than the cost of a Modi/Magni combo you can get something like the Focusrite Scarlet 2i4. Which not only solves the 'handy volume knob' question it also offers analogue line ins for your other gear, pro balanced connection (+6dB, isolation from interference), MIDI, a powerful effects/EQ package and the option of two separate stereo line outs. Which you will soon find indispensable.
http://uk.focusrite.com/usb-audio-interfaces/scarlett-2i4/specifications
There is a huge range of suitable units at all price points. Take a look here.
http://www.thomann.de/gb/cat.html?gf=usb_audio_interfaces&oa=pra
