How many watts per channel are enough?

Your far safer using a bigger amp than the speakers can handle because now you have the luxury of using only SPLs to determine maximum speaker output, ie: pain threshold:)

Contrarian Assertion is the name of my old dude punk band.

Use nice, round figures...50,75,100.

I have a 34 WPC PrimaLuna Prologue 5 power amp. I have found that because of the ability of tube amps to distort musically, 34 watts of tube bower sounds more like 75 watts of solid state power. The amp will distort at or above 34 watts as rated but unlike transistors that hit a brick wall at their power rating and sound terrible above it (and can destroy your speakers), tube amps distort 'gracefully' and don't clip (or at least don't clip as abruptly).

I've had a pair of Quicksilver Mid Mono amps for about three years and have had them paired with several speakers .  With Revel M106 at 87 dB sens they sounded great with 40 watts at moderate levels but crank it up and they lost their composure fast.   Same speakers with a McCormack DNA 125 and they would play loud without strain.  

Klipsch Heresy III with 40 watts is amazing, dynamic  and effortless at all volumes.    I have used them with a B&K 2140 and they sounded ok, but no contest to how the first few watts of EL34 goodness sounds.