First, you do understand that he is using a musical metaphor? Each intellect, in m'Lord's opinion, is an instrument, and combined, perhaps, you may have an orchcestra. First he gives credit to the British as being the better part of that orchesta, or would be a better orchestra on their own, but then suggests that the illiterate (uneducated) among the British, play a drum. I think he feels that the drum is not much of an instrument (I know many drummers who would disagree!), that you simply pound on it. He acknowledges that an orchestra needs a drum, but that it is the lowest (most bass? pun!) of the instruments.