Totally agree. I was talking to a friend today about amp circuits. He said it takes him years of tweaking a circuit to get it to sound the way he wants. A tweak can be a different cap, resistor, even a tube. It all matters. And I have inserted a tube and hated what I heard. Heck, even a fuse can be heard.Replacing components by more expensive components can either improve or ruin the sound of equipment. The sound of a component in a circuit is associated to its function and the circuit itself. Using other people opinions in different equipment is a risk.
In an hobby ruled by user preference DIY benefit from strong bias and accommodation factor - as we listen to partial changes we get used to the modifications. I have sometimes carried such upgrades in expensive equipment, but always advised by the manufacturer.
IMHO sending the VAC equipment to the manufacturer is an excellent idea.
I would most definitely send it back to VAC. Its a good amp and preamp. Worth having the manufacturer tell you what they can do. You might ask them what 300B tube to use.