First of all:
1. Observe systems synergy! Speakers that are high current / high excursion, demand specific amplification, and the experience will not be a good one unless the amp is specifically designed for that task.
2. Tube amplifiers operate with output tansformers. The taps of the output transformer define which speakers they are designed to drive. Hence, it is vital to get the right match. (Eg right size shoe for the foot - if not, you can't run, or worse, it will even bleed..)
3. Watts define only what is the RATED peak output of an amplifier.Also, the ratings apply for power into a RESISTOR. Solid state amps can put out LESS than the rated powr into an actual speaker, while tube amps put out MORE into an actual speaker than into a dummy resistor used for rating. A push pull tube amp with lots of feedback only slightly more than rated, but a zero feedback SET can put out even up to x10 the rated power into a real loudspeaker.
However, the wattage only implies PEAK SPL capacity.
In every amp we are going to observe problems before that peak spl is reached.
Solid state amps show signs of strain already below 1% of their peak wattage output. Single ended triode amps show strain around 50% of their rated peak output, provided their power supply is adequately designed (many commercial models are not - not the fault of tube technology, but result of cutting corners to make sales).
Also, we cannot generalize tubes / solids...
Among solid state amps class A low feedback designs can handle much higher output without distortion, so for example, an 50W class A solid state amp can put out about 0.5 clean watt. (Clean enough for my personal demands.) For a class B solid s amp, the clean power is much lower. (I found it's non-existent, but others will find a few adequate watts there...)
For tube amps, the clarity of watts is defined by the power supply, which you will not find out based on the watts number.
For example, a tube amp rated for 1W output but with a power supply that is designed to supply a 300W demand will be able to deliver a cleaner output than a 200W class A solid sate amp. Of course, the 200W amp can go much louder, but the perceived quality at the 20-200W range will be much inferior to the milliwatt to few watts range.
In short, when going for watts alone, then usually:
1 SET watt = 10-100 solid state watts (provided the SET has ample power supply - if not, it might not even be 1 watt..._)
2 Push-pull tube watt = 5-20 solid state watts
Also, provided the tube amps are matched with speakers that they were designed to drive. (Eg have the appropriate taps! )
We often hear complaints that tube amps cant drive a high current speaker, because its impedance drops to 1.7R or something similar low value. The issue is not with the tube technology, but those "conoisseurs" who are so clueless they want to drive 1.7R speakers off 4R and 8R taps.... Talking about trying to squeeze in a watermelon into an orange juicer! Such loads need a tube amp that has a 2R or even better, a 1R tap... that's another story, but something that has to be clear to everyone who is new to tube amps and wants to find the right tube amp match.
BEFORE YOU LOOK AT WATTS, MAKE SURE THE AMP HAS THE RIGHT TAPS TO DRIVE YOUR SPEAKERS. And not the brochure specified bogus number. The actual measured real impedance, which is specified by the dip.Sadly, a lot of manufacturers nowadays lie shamelessly, calling even 2-3R speakers as 8R...
When a loudspeaker is advertised as: "This is an 8R loudspeaker that dips to 2R at 120Hz" , then that means that it's a 2R loudspeaker, not 8R! You need a 2R tap to drive it, 8R tap will be entirely completely insufficient and utterly useless. Not because it's an inadequate tube amp by itself, but because the delinquent is choosing the wrong shoe for the foot, so to speak.
As in life, not all shoe fits all feet. Just because it does not fit, does not mean it's poorly made shoes, and would be terrible on the feet of those whom they were made for.... systems synergy is all. ;