Trying a different brand.
I'm no expert at this, but I know some amps allow you to roll different types of power tubes, which may require re-biasing or even changing internal circuit values- not something I'm familiar with. I once had VRDs, but never rolled power tubes. Instead, I played with the 12ax7s. If you have a tube preamp without tone controls (as I do), you might try playing with the signal tubes (usually a 12a type or 6dj8) FIRST, before you play with the amplifier. That gives you the most change, IMHO, without breaking the bank.
But beware, the tube rabbit hole is deep!