How much power needed to drive 91db speakers?

It is a combination of the voltage ease which 91dB @2.83V/1m is very easy load voltage wise but what is the electrical phase shift like? That can mean you'd need a high current amp. But yes plugging these parameters into this calculator - 

 

Even if you sit 12ft away, you'd be getting to 96.7dB SPL peak at your seated position with the 50W maximum tube amp.

 

But why do you want the tube amp? You already have a competent power amplifier why cripple yourself? @curiousjim 

I think that evaluating specs is an exercise in futility. I had an integrated specced at 300w @8ohm and then moved up to separates with an identically specced amp. The separate amp must be making twice the actual power of the identically specced integrated. Get some gear into your room and trust your ears. I don't trust the measured specs.

My room is X feet long and Y feet wide and Z feet high.  What color should I paint it for best sound?  Pretty absurd question right?  Please try to think before you post.  Ask yourself what is it you really seek to learn.

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.