Is 15W enough to drive Dyn 3.3?

Follow your instincts. :)

I do not doubt they will sound pretty good, I just think you will not be able to get the most out of them with that little power.

Good luck.

JT-Thanks for the comment. Does anyone know what's the minimum Db (for speakers) that performs well with a 15W SET tube amp?

Email the folks at Dynaudio. They are very serious about their advice and they know their speakers well. Dale

I recall the documentation that came with the Dynaudio Contour 3.3's, suggesting a minimum of 100 wpc.

If your room is large, you need at least 95dB sensitivity to approach the dynamics of live music with only 15 watts. In a smaller room the sensitivity matters less because of the room reinforcement.
Dynaudios have around 87dB sensitivity, and the 3.3s have several drivers, an additionally taxing load for an amp.