Start with the stock tube to get a baseline feel for the amp. Then the two tubes you should try first, which are also pretty cheap, are the Voskhod rocket 6N1P-EV and a 1960s GE 6CG7 with center shield. If you get adventurous and don't mind running the risk of a slightly noisier tube (it's like rolling the dice), go for a 1970s or 1980s GE 6FQ7 without center shield and "hot wire" series filament heater wire. This is also pretty cheap.
GE tubes get a very unfair bad rep around a lot of tube parts, but that's good for true rollers because it keeps the costs down. GE did have some duds, but every GE tube I've used in my all my amps have been pretty neutral with good air and detail. Back in the day, they were viewed as premium tubes before collectors starting getting interested in exotic Euro tubes. It's a shame GEs are no longer remembered as premium.
You'll find something you like in that bunch. If not, you have to go further up the cost ladder, which may be kinda foolish in such a cheap amp. Let me know how you like any (or don't) any of those tubes and then we can discuss more expensive options. But, as I said, this could be foolish because, even though tube rolling does have an effect on the sound, it's not a drastic, night and day difference between tubes like you may find in other amps. You can never fully escape the Vali 2+ DNA, which is neutral and a bit forward, especially in the treble region where you get snare drums and high hats. Tubes can tame that a little or make it even more aggressive to your liking, but you're never gonna get a warm and gooey OTL sound like a Woo Audio WA3, for example.