Crubio,
As with any fine tube amp, the effects of tube rolling in the Doshi preamp are very audible. Which tubes you'd prefer are of course a matter of taste.
That said, Nick designed and continues to upgrade the circuit to perform optimally with current production, readily availabe tubes. You can chase costly NOS tubes if you wish but it's in no way required or suggested by the Alaap's design or designer.
My preference is for tubes with the sturdiest, least resonant internal structures. This minimizes tube "bloominess" and solidifies dynamics and bass. Having tried a variety of tubes, I prefer the best quality current production models from JJ or ElectroHarmonix. No showoff factor. No impressing anyone with exotic, only pair in existence rarity. Just clean, extended, tight frequency extremes with a harmonic/timbre reproduction that sounds realistic to me.