I would think that how contemporary EL34 tubes sounded in the unit would be one of the design elements in a very well crafted contemporary design.
If you build a tube amp with a contemporary tube and change all the other design parameters to optimize it for the sound you want... with circuits, capacitor, resister, etc choices... then NOS would just sound different, not better. Assuming of course you want the sound the designer produced.
I'm sure that many mid level designs are made expecting tube rolling and therefore are fairly generic middle of the road design... an let the user tune with tubes.