I confronted the same issue when I was shopping for 300b tubes for my New Audio Frontiers amplifier which is often compared favorably with Mastersound. An amplifier can operate with a range of tubes but will only perform to its potential if the numbers line up. Your dealer probably knows the performance parameters your amplifiers requires of a tube. A modification could work for you but its chancy - every component in your amplifier was chosen for the way it works with the other components. Good luck.
Mastersound Compact 845 with Pvsane 845T
I own a Mastersound Compact 845 and Mastersound PH5 Pre Amp and orderd two Pvsane 845 T Valves.I was advised by the valve supplier that they would work ok as they were the top of 845 range of valves.I was then advised by my dealer that a mod had to be done to the Cathode Resistor on the Compact 845 to take account of the voltage increase.Otherwise the uit could in the future fail.Any body out there using the 845 T in the compact without modification to the resistor should be aware of this .Any comments welcome