Apologies! I was quite tired when I wrote my original post.
Basically, adjusting the Windows slider = digital attenuation. What you're doing is reducing the volume by effectively reducing the number of bits, thus lowering the total volume output in the end. So yes, the signal would be softer. However, this does mean as soon as you use any kind of digital attenuation, you've lost bit-perfect output period. In a very well-designed, well-implemented digital attenuator, the result might be audibly transparent, but bit-perfect it will not be.
To make things more complicated, bit depth is only part of the equation. All digital attenuators use different math to reconstruct the signal between subsequent attenuations, and some (i.e. most) implementations are definitely audible, the Windows and Mac ones definitely included. What typically happens is that the music loses information; it sounds more "gray," uninvolving, and lacks details. It does not require anything close to a TOTL system to hear this; I heard this effect out of a Grace SDAC and a Schiit Magni 3 with an unmodded HD6XX. I wager this is audible even with off-the-shelf computer speakers (assuming of course that the speakers are active).
So my advice is to avoid this if at all possible. But to answer your original question: yes, the signal coming out by lowering the slider will indeed be lower.