How much power needed to drive 91db speakers?

You get 91dB @ 1m from 2W.  It will drop by 6dB at double distance resulting in 79dB @ 4m.  Presence of the second speaker and room reflections add 3dB each resulting in 85dB.   20W of power will produce sufficient 95dB loudness (85dB+10dB).

@kofibaffour 

Opting for a tube amp is hardly “crippling” oneself.

I regularly drive a pair of JBL 4349s (91dB) with either a Sugden A21 SE Signature (30W class-A solid state) an an AR i/50 (50W push-pull tube, class AB). No issues with either having plenty of power.

To the OP, I have several pairs of SF speakers. You may want to look at the impedance curve. Many SF speakers have a very low dip in impedance at the low frequencies. The current output may be more important then the power output in watts. 

Sonus Faber speakers are a wolf in sheep's clothing. Starting with the mid-level models and up are very watts and current hungry but just as sensitive with electronics and cables. SF work very well with MAC gear and sounded sublime with Burmester.