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. 
Ah, yes the "magical midrange." I agree. Yet in my Blackburn, the mids and upper mids seem over-emphasized. I prefer to tone it down a notch or up the bass and midbass. Could be a mismatch with my system, I suppose. Does the Mitcham have as much detail and air? 
I use RAM 6922's in my ARC phono amp and they are by far the best tubes I have ever used. 
Wow, it took a long time --150 hrs or so -- for my Mullard e88cc to break in. Now the bass is fleshed out, the highs smoother and soundstage more dimensional. Altogether a more balanced sound. Yes, magical, richly satturated midrange, if without the razor sharp delineation of the Siemens -- a wetter, more romantic sound. It's certainly a keeper, at least for certain kinds of music..