Within 4P1L tube's there's not much variety to look for. Scrolling trough other forums you find that some have tested a lot of 4P1L's and a trend may be found with older 4P1L measuring better. Other then that, don't know.
The only 4P1L i'd look for is the one with the least microphonics.
For better sound from a Smash, there may be some parts that have an influence on sound that can be improved.
The shunt regulator Smash-up upgrade will make the biggest difference. Maybe there's something to be gained by improving the filament supply?
From what i've read with other amps, the shunt tube will make a minor or no difference in sound.