Thats funny caused i just tried it your way and my Main Volume Level was at 100% just like i had set it with my Teskbar Icon.
As a matter of fact almost all my levels were at 100% except my Line in and CD Audio. I never opened up the porperties and just raised the level with the icon. So it can still be done. Why cause that icon does CONTROL the Main Level. That is its purpose. You say to use the properties to raise the main level which is what that icon controls.
That little sound icon is what i am talking aobut. That controls the main volume levels for your PC in VIsta. If you raise that it does the same thing you stated to do.