The British Sound

If anything it is more likely that British homes are smaller and the treble is what attenuates most with distance. The closer you are to the speaker the less treble you need.

The other possibility is the Boom Boom Tizz factor - simply put there is huge competitive pressure on American designs to get the speaker to sound as impressive as possible. The bass and treble are how most designs are evaluated and compared.

A thought from a different perspective. When there is high barometric pressure pushing on the eardrum treble clarity is reduced. During periods of low barometric pressure treble clarity/harshness is increased. Next time you are listening on a sunny/high pressure day just equalize your ears like you would when you fly on a jet airliner or scuba dive (hold your nose, close your mouth, and blow outward). You will experience an increased level of treble extension. As to your question: the British sound is the result of studies commissioned by the British government as to how to make natural sounds, not laid back, mellow and un-aggressive. The result was the BBC monitor produced by companies like Spendor, Harbeth, Sterling, etc. Upfront, etched and aggressive sound is the trademark sound (regardless of nationality) from manufacturers trying to create something which is actually unnatural but distinctive in order to attract customers, thereby creating the need for AudiogoN.

Brit speakers have evolved over the past decades and
The mellow tag is less applicable today. in fact box speakers have improved to the point everything sounds remarkably similar.

Part of this is due to the fact that British ears are voiced differently then other cultures. Many years ago an excellent article appeared in FI magazine that covered the issue in great detail. So, yes there is a British sound, tuned to British ears.

The affect of humidity on sound waves is not appreciable. Who here actually uses a humidity meter? I have one hanging on the wall of my listening room and I check it periodically out of curiousity, but I am not convinced the sound is better with higher or lower humidity.