What exact tube amp are you using? Knowing the rest of the equipment will help as well. Also, how large is the room the system is in, what is the placement, etc. Vandies do need power to perform their best, but if the tube amp can deliver you shouldn't see any problems...
My money is that you actually have a speaker placement issue, more so than an equipment issue.
My money is that you actually have a speaker placement issue, more so than an equipment issue.