Two points: 1 - The technology of the LS-50 simply did not exist when the LS3/5A was developed - using KEF drivers, if you recall. Time moves forward.
2 - Any power amplifier will drive any speaker. The question is how loud. In the case of the LS-50, it's sensitive 85 dB 1w/1m. A 20 w amp will drive them 13dB above that - 98 dB. 2 speakers adds another 3dB, so that combination would yield a peak SPL of 101dB
at 1M. KAllowing for 12 dB of peak headroom, that means you system would support average listening levels of nearly 90dB, or what most would consider quite loud. Doubling the power adds just 3dB, a just noticeable increase.
With your requirements for desktop use, the KEF LS-50 and 20W/Ch tube amp listening a less than 1M should work quite well. Any LS3/5A derivative will be 2-3 dB less sensitive and be subject to the driver integration issues several others have already discussed.
I have the original LS-50 in my small office, driven by a 20W/ch Class D amp, with a Bluesound Node source. I am very pleased with the results. In that small of a space, the lack of the lowest bass octave is hardly an issue.