Watt Gives?

An amp's ability to deliver current, especially into 4 ohms and lower, is a better indicator of its performance with different speakers than its voltage rating!

Good YouTube video. Paul McGowan explains it all.

Partially it's clipping and partially it is the poor damping factor of the tube amp screwing up the frequency response emphasizing some frequencies to give the impression of louder. 

  An amp's ability to deliver current, especially into 4 ohms and lower, is a better indicator of its performance with different speakers than its voltage rating!

Agree. I have always enjoyed using amps with big power supplies that can deal with very low impedance speakers (one of my favourites puts out only 40 watts into 8 ohms, but when bridged will put out 500 W into 1 ohm).

As it happens, I still own one of Belles rather rare amps, the 'Belles A' - I believe that he made less than 200 of them.  70 W output, but when asked about how low impedance it would drive into, supposedly commented along the lines that t would drive a clip wire (between the output  terminals). What I do know is that the turn on pulse welded a couple of switches together so I just used a commercial power bar to turn it on (and replaced the bar when it eventually failed).

Tube amps can't do that - but when they do clip, they do it more gracefully.

The reason is that new solid state amplifiers are low current devices based on cost cutting when you listen to a good vintage class ab solid state that is 250 watts with a lot of current behind it the 75 tube watts will sound not nearly as dynamic but you need to listen to a solid state amp made before 2000 to get what i am talking about.