The less parts in the signal path, the better
The advantage of tubes is largely that they're NOT transistors. If an amp can use a tube rather than a transistor, then it's a good thing..
so basically what you want is an amp that can replace all its silicone with tubes, but using as few tubes as possible.
NOTES:
1) this is from the perspective of a person who prefers tubes to transistors. some people prefer transistors to tubes, so they'd want the opposite
2) this is from the perspective of a person who is of the philosophy that "less is more", which is common in tube fans
3) In reality, it's not that easy to tell. Just because an amp has less tubes, doesn't mean its better. It's a very very rough guide at best