"The 7236 is our overall favorite, but it is difficult to find."
http://www.pmillett.com/Wheatfield/headphones.htm
I've got to believe the amps XFMR's are capable of feeding the 6336, 6528, and 7236. Pete is a bright guy, and you can bet he did the math before making the tube recommendations. That being said, the amp does run warmer with the 7236 than it does with the 6080. The 7236 also gives the amp a more tipped up top end than the 6AS7 and 6080 which is not complimentary to the AKG K701 or Grados IMHO.
Also see this link for comments on headphones for use with the Wheatfields:
http://www.pmillett.com/Wheatfield/headphones.htm