I suppose the first question to ask is whether you were satisfied w/the tubes you are replacing. The second issue is tonal balance, as each brand supposedly has a different one. In researching tubes on Audioasylum and other sites denizened by tube heads, I've concluded that the biggest determinant of how a given tube sounds is the circuit (and bias level) in which it is used. Finally, are you sure that this amp is limited to these types, as some amps that accept these can also accept EL34s, supposedly a warmer, yet slightly less powerful tube.