my observations are corroborated by many others,
Well, MY observations are corroborated by many others, e.g. when I was a teenager, whenever I visited my friends in Prague, they would go absolutely gaga over Polish boys. Also, my female Czech friends of various ages have often commented that Czech men have no testosterone and make lousy partners.
the phenomena being so noticeable.
Well, I for one haven't noticed it.
Germanic influence on Czech republic (cultural and genetic) is indisputable, you should know that as a Czech.
The Germanic influence on Poland (cultural and genetic) is indisputable, you should know that as a Pole (flagless or not).
Heard quite a few opinions by Polish girls visiting for example Berlin as to the rather perceptible spike in male attractiveness over there.
Of course they found them attractive - because German men tend to look and act more feminine than Polish men. Read my post above about Polish women and what they want in a man.