I was amazed to hear how different the 'American Beatles' sounded and why, years ago, people in the UK were always paying to get the American imports. (exact opposite phenomenon to the States obviously)
The 'Capitol sound' (different EQ, compression,extra reverb/duophonic and all that) gives the early stuff a super-futuristic 'Beatlemania' feel and, in a way, creates this parallel band that The Beatles were quite shocked to find themselves having to become a couple of times a year when they went to tour in the US.
'The Beatles' was a monster beyond the band from the beginning in the USA. They had to wear the suits of what American Beatlemania and Capitol had done with their stuff and all of this probably went into the frame of Paul's eventual Sgt. Pepper alter-ego idea.
Compared to the bright sound of the Capitol stuff, the Parlophone stuff is more studied, has 'northernness' written all over it and is more focussed, less hysterical.
Around 'Paperback Writer', the two sounds fuse as the band begin to expand to fill their US image and the Capitol sound and then go further beyond anybody's take on technology and anything else.
The 'English Beatles' are primitives and experimentalists, the 'American Beatles' are sugared up, hyped up ultra-commercialised uber-versions of themselves which they eventually surpass.
The difference in feel and sound between the two is surprisingly immense at times, with the same source recordings and hearing the Capitol boxed sets in the last couple of years finally gave me access to a whole other Beatles I hadn't really known about.