I have a reverb with CV control of Mix and it really is less useful than you would think. What I prefer is to have voltage control over the "send" level. I achieve this by placing a VCA before the reverb and a mixer after, but I think Doepfer makes a module specificly for this (FX/Crossfade, or something like that).  The difference is that when you turn the mix from wet to dry, you are lowering the volume of the reverb tail. If you instead control the level going IN to the reverb, then you hear the entire tail.  Try doing this on a mixer with send and returns.  First manually adjust the return with the send up, then try adjusting the send with the return up.  Which do you prefer