Best 845 Tubes For Art Audio Carissa?

I need to replace the 845 tubes in my Art Audio Carissa Signature and am not certain what will work most effectively. I'm aware of the top rated tubes but need additional information from another owner hopefully. Before my unit was shipped to me, Joe Fratus, former owner of Art Audio until it closed (and the brilliant designer of their products) told me that he went through ten different 845 tube brands/models before finding one that was up to snuff for use in the Carissa. I just don't remember what was technically lacking in the others but it was a technical limitation of some sort. Unfortunately, the tubes in it now are no longer available. 

Any help would be appreciated. Thanks.