Roger can give a better answer, but my recollection is that the V2 tubes are basically drivers and can be most anything with respect tu tube grade. The V1 tubes as I recall it, however, are an order of magnitude or two more important in the amplification chain. As such they should be the lowest noise variety that you can find. They also must be closely balanced between the two halves of the triode. I cannot recall if you call it DC balance or DC offset, but this parameter has to be pretty close on the two halves in order for you to have enough range on the trimpot to null the LED out thereby getting the lowest distortion. Many NOS tubes are not close enough to null out, and even the RAM tubes can drift out of spec in this regard as they age. The latter issue sounds like the one that you are dealing with with your amp.
