The only thing a mixer really does is allow you to mix the outputs of various devices - mics, etc... The output is essentially a line out voltage, just the same as your keyboard. The volume control does not really increase that output voltage, however, it does allow you to vary it from low to maximum line output level.

I sold all my mixers, mainly because I no longer play with a band and have bandmates plugging into the mixer with their instruments. I plug my mic into a TC Helicon Harmony-M and the output of the TC goes into the mic/line in of the keyboard, which is set to mic. Yes, the TC output voltage is line voltage as well.

I have never measured the output level of my keyboards, but I have measured the output voltage of my mixers. They were all at line level, which is 1-volt RMS.

Gary