Winggo, much more important than your speakerss sensitivity is its impedance. If you have low efficiency speakers with a benign impedance, you can solve any power issue by adding more tube power. If your speakers have either too low or too demanding of an impedance curve, then tubes are not recommendable, regardless of power.
http://www.enjoythemusic.com/magazine/bas/0808/
http://www.enjoythemusic.com/magazine/bas/0808/