Ian Stewart was the 6th Stone. The pianist on a lot of British invasion records is Nicky Hopkins, heard on albums by The Kinks, The Who, Led Zeppelin, The Jeff Beck Group (I saw him live with Beck on the first U.S. tour), The Stones (Satanic Majesty’s Request, Beggars Banquet), and even The Beatles ("Revolution"). In the 70’s he moved to the States and worked with The Quicksilver Messenger Service and The Jerry Garcia Band. He made three solo albums, and died in 1994.
Steve Hoffman has posted pictures of Abbey Road studios in which the recording equipment is visible, pointing out the tube limiters the studio used. Steve claims those limiters are known to be not very good sounding, and may very well be responsible for the less-than-great sound of The Beatles albums.