How many watts per channel are enough?

Consider the fact that often you can save some money by buying the lower powered version of a well regarded design and get the benefit of a greatsounding amp without extra power you likely don't need. Pass Labs makes that point all day. And remember, a tube amp that is pushed in louder musical transients can still sound sweet as the character of the distortion (2nd order) is more ear friendly than a solid state amp clipping, which is a sound nobody should have to endure.

Buy a Lamm M1.2R, that's something serious and you can close that chapter.

Rule of thumb applying the correct amount of RMS power to a loudspeaker. To get the very best performance from a loudspeaker as explained to me twenty years ago by a speaker designer, find out what the maximum RMS power the driver magnets and the crossover capacitors can sustain over the long term of continuous power and divide by two.
If the speaker has a max of 500 watts rms continuous then use a 250 watt amp. There should be at least a 50% volume of continuous current to the magnets. By filling the magnets continuously with current volume at 50% or higher, results in cooler temperatures to the voice coils and crossover network. When you under power a speaker and drive up the volume, you increase the chance of clipping the amp and over heating your drivers causing the coils to fry. If your using a 60 watt tube amp, Match it with a speaker that has a max of 120 watts rms to get outstanding performance. There are a lot of low power full range speakers that are perfectly suited for tube amps.

You sure that isn't "multiply by two"?

Why would you multiply by two? The formula is to divide in half by 50% or 2 into the total amount of rms wattage
the speaker allows. If a speaker has a max capability of 1000 watts and you multiply by two, thats too much current
at 2000 watts. Goodbye Charlie. You just fried your speaker with too much power. You can damage your speaker
under powering by clipping your amp or too much power. However, in most cases most amps are much more bullet proof today with built in safety shut down thermal switches if clipping occurs compared to thirty years ago.