Why tube amps are rated so low, still don't get it

Since in general tube amps have a large low freequency slope bellow 70Hz, they seem to sound more loud than SS amps.
You may figure that 50W tube amp will do only half-power bellow 60 Hz and a quarter power @45Hz...50Hz.
So the meaning of "Monster" is realy a misconception since most of the power in this case is being spread from mid-bass to higher freequencies.
Music or audiable freequencies bellow mid-bass require the most amount of power... let's say 70...80%. SS amps can work from DC to a very high freequencies thus spreading the same amount of power over much larger freequency bandwidth than tube amp. In case with SS 50Wpc, 40W may go to the bass freequencies and only 10W will be left-over for the rest of audiable bandwidth while in tube case everything goes right-above bass freequencies and only 25...30% down-bellow...
Such slopes are due to the output transformer that is already itself acts as a high-pass filter. The quality of a low freequency performance and so is overall performance of a tube amp(non-OTL) largely depends on an output transformer.
The linearity of a tube amp is also being altered by speaker load characteristics that are in most cases very complicated.

All we're trying to explain is delivered power to a visible by amplifier freequencies.
A noise contains all components of such audiable "visible" freequencies.

If we take a trivial sine wave 1kHz and feed to both 50W tube amp and 50W SS amp they will both deliver the SAME power to the dummy load(measured by true-RMS Voltmeter).

Speaking of the noise noise experiment we may assume all audio freequencies but far not all load combinations.
We also can apply the term of "visible" freequencies and in SS case they're much larger let's say at particular THD limit set for both experimented units.

Now, in your case when you're shopping for full-range speakers of medium efficiency... I'd have to say you'll only hear half of them with 50W of tube power. I'm not saying that you shouldn't shop for tube amp that HAS plenty of advantages to the SS but you'll have to shop for higher powered ones than 50Wpc.
You must also take a note that higher power tube amps have a higher-complexity power supplies and so with cost of parts and lower reliability.
I have a tube amp myself and it took me a while of research how to improve a reliability without loosing performance and I finally did it at once. I now prefere in bi-amp setup together with Sunfire Symphonic Reference unit fed trhough DIY balanced converter with adjustable gain -10 to unity.
It saves the tubes and brings me the best of both worlds of amplification FYI...