With 100db speakers, you could go for a pretty low powered tube that wouldn't give off much heat. Are you concerned with too much heat in the room overall(not a worry) or someone touching it and burning themselves? Plenty of designs have cages that would prevent that.
IMHO, if best overall SQ with those speakers is the priority, class A is the way to go.
A couple to consider: Air Tight ATM1, Mastersound Compact 300 or Compact 845, many Shindo models
Cheers,
Spencer