LETS COMPARE TUBE AND SS AMPS. TECHNICAL

First of all the Almighty set ALL watts to be equal to the product of Voltage and Current or physically Work per unit of Time.
The power P that is measured in Watts can be consuming and giving and the remaining power dissipates on heat.
Tube amplifiers and Class A SS amplifiers are only (approx)20% efficient since the remaining 80% of power goes onto heat.
The mentioned 'sonic watts' can be described as SPL that is measured in dB(usually guitar dudes know what it realy is). Decibel is a logarighmic relative value that compares no sound to the actual sound. SPL can be measured also at particular freequency and that's where all the Tube/SS confusion burried as the 'Holy Grail' for some folks arround here.
Normally tube output devices have very high output impedance and amplify voltage that suppose to be converted to current using the output transformer to satisfy nowdays speaker voice coils. Transformer is the device that converts variable electric field of an input coil into the variable magnetic field and the output coil has the induced electric field from the variable magnetic field. The input and output coils of the transformer are actually inductors that have variable impedance depending on frequency. As the frequency drops, the reactance is minimal and impedance of the inductor approaches actual resistance of the wire. In this case the voltage accross the input coil terminal becomes MINIMAL as well as magnetic field across the primary coil! Another words by nature there's hardly any lower-end extention and 'visible' spectrum of it is substantially less than corresponding SS amplifier. So placing 'in front of each other' 100W tube amp against 100W SS amp obviously SS will seem to sound more quiet, but if measured SPLs across the 'visible' frequency spectrum, SS will have it larger in both ends of a slope.
As to the speaker voice coils, lower frequencies should be hadled with much higher current than midrange.
In tube amp transformer would act rather as high-pass filter thus distributing a larger portion of an output power onto substantially smaller frequency range giving the 'illusion' of so called 'TUBE WATTS'.

First off, power is only important if you need it. Meaning, that if you are using speakers with a sensitivity of 105db, you could run them with a transisitor table top radio. You would only need about 500 milliwatts, half of a watt. But if you have maggie 3.6, or Sound lab, then you need a few hundred watts, like 500wpc into a 4ohm load. The load presented by the speaker is important, and the sensitivity also is impoprtant. Finally, they need to be synergestic, less concrete, but very real. I like well matched SS and well matched tubes. A friend has the latest legacy whisper speakers rated at about 96db sensitive, and we compared a 70wpc vs a 600wpc SS amp. Which had the best bass??? The tube amp, best mids, tube amp, best treble...you guessed, tubes. This SS amp is said to be a great match for the legacy whisper, but tubes beat it. All tubes won't as my quickies were a bit slow on these speakers. they are great with mine, 3.6's. Factors like damping factor, speaker sensitivity need to be included in the consideration of amplification. I prefer tubes, but cheap good watts are also available with SS.Jallen

Your system, speakers and room combined with your listening habits will matter most. Cables and power delivery can also make a HUGE difference and do not have to cost a fortune. Any comparison of a tube product vs ss will be mute without the rest of the picture filled in:O)

Lets check the archives and say we did.