How much power needed to drive 91db speakers?

@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. 

Congratulations on owning such fine speakers.

I have a pair of Kef Reference 3-2.

91dB efficiency at 4 ohms.

I drive them with a Conrad Johnson LP66S tube amp using 6550 tubes.

It delivers about 70 watts per channel in push pull mode.

My CJ preamps volume control goes up to 100.

If I push past 50, I most likely will blow the windows out of the house.