So I decided to give the Dynaudio HS speakers a whirl with the Conrad Johnson amp which is 125 watts per channel. I didn't expect much, but was sort of blown away. The CJ amp totally opened up the speakers in a way I didn't expect and made me realize I can get away simply powering them with the CJ amp.
@bluethinker The difference between 100 Watt and 200 Watts is 3dB, which to the ear is barely noticeable. As you found out this is really more about how the amp plays with the speaker than it is power.
There are some things to consider when using tubes with 4 Ohms. First, although tube amps have 4 Ohm taps, the output transformer often loses an octave of bandwidth between 8 and 4 Ohms because its less efficient. This loss, which is usually less than 20Hz, results in less impact due to phase shift in the bass. If you measure the temperature of the output transformers with a thermal camera you'll also see they are running warmer into 4 Ohms- and that heat is power made by the output tubes.
You'll also find that the speaker cables are far more critical using 4 Ohm speakers so its best to keep them as short as possible (an argument for monoblocks). This is particularly true of tube amps which tend to have a higher output impedance.
There are class D amps now that don't take a back seat to tube amps of any kind in any way (except overload character) that can make the power you were asking about.