Mitchum or Blackburn for the Mullard 6922?

Do Mitchum production and Blackburn production sound different in the Mullard 6922? I believe I once read somewhere that Mitchum made tubes for the military, whereas Blackbutn did not, so, presumably, with better materials and/or tighter tolerances or testing. 
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He's just a little north-west of Croydon, between the A23 and the A24.  Where the best action movies are made.
Go to and buy a set of their super low noise tubes and you will never look back.