I have the YCS50 head. Yes, it does lower the volume with the voice on. I like it with the voice OFF as it seems to make the amp sound more open without using the resonance and presence controls, but that's just me.
Also, if you engage the "Brit" setting on channel 2, you may notice channel 1's gain bumps up a little more. Not sure if that was intended in the design, but just something I've noticed. I've had my head for almost two years and is my main amp and love it. I installed JJ KT77's and love the sound over the stock EL34's. Congrats on your amp!