McIntosh C220 tube question

Has anyone tried any tube rolling with this preamp? According to the manual, tubes should only be replaced by an authorized technician. I've changed tubes in my previous preamps with no problem. Is there something that makes the C220 more difficult? If you have done some tube rolling, which tubes did you experiment with?
search this forum:
Well, let's try this again. Has anyone tried any tube rolling with their MC220? If so, what did you find worked the best? Thanks.
read my review of this preamp on . i replaced the tubes, with hardly a noticeable difference in performance.

if you speak to the technician at mcintosh, he will tell you that the sound of the preamp is independent of the tubes used.