Power and volume have very little to do with one another. Sure if you drive the amp flat out then it will compress quite badly, same goes for the 006t since the 6CG7's are run past their limits. What you are descibing is just gain which can easily be increased or just convert the amp to a balanced input to double the input voltage. I'm all for more power but it is all about control over the diaphragm and load tolerance but not volume.
Converting the amps can be difficult but some are very easy. Solder two wires from the transformers to the PCB and change the jumper settings. Minimal skill involved.
As for the US warranty, that's a none issue. There is nobody to service the amps should they fail as Yama's doesn't answer the phone.