Saxo,
It sounds like you are trying to talk yourself out of upgrading the tubes. Stick with what you have if you are happy. If you find you want a little more (as most of us do) then I highly recommend upgrading the pre-amp (small signal) tubes first. It does take some research. Different tube types, different manufacturers, different amps. My pre-amp only has 4 tubes so I replaced the entire quad with a matched set from a reputable dealer here on A'gon. There was a dramatic change in sound. All the good things you read about NOS tubes. Try to find ones which are matched, tested for noise and microphonics.
I'm sure that amp designers would use Mullards, Amperex or whatever NOS style tubes if they were currently being produced. I think that MF bought up the world supply of Nu-Vista tubes so they could make a limited production run. They simply can't sell a product where the tubes are in increasingly short supply.
It sounds like you are trying to talk yourself out of upgrading the tubes. Stick with what you have if you are happy. If you find you want a little more (as most of us do) then I highly recommend upgrading the pre-amp (small signal) tubes first. It does take some research. Different tube types, different manufacturers, different amps. My pre-amp only has 4 tubes so I replaced the entire quad with a matched set from a reputable dealer here on A'gon. There was a dramatic change in sound. All the good things you read about NOS tubes. Try to find ones which are matched, tested for noise and microphonics.
I'm sure that amp designers would use Mullards, Amperex or whatever NOS style tubes if they were currently being produced. I think that MF bought up the world supply of Nu-Vista tubes so they could make a limited production run. They simply can't sell a product where the tubes are in increasingly short supply.