The Mellotron Group

Hi all,
One thing that has puzzled me for a while that someone might be able to answer is that on early genesis live recordings eg the 70's box set the brass sounds completely different to the MkII brass I have on my trons. And yet I'm assured it's the same sound as used in all MkII's. If you listen to Willow farm for example or that bit immediately after it, it sounds to me like some kind of nasal buzzing/honking rather than brass. I can't replicate it at all. Is this how the brass did sound through MkII heads and tube preamps or is it something else?
John