I haven't tried them yet. They're for my YBA-1 which I typically run through a load and reamp. That would be a huge waste with these so I'll save them for times I want to unleash the beast and run loud and proud.
I expect they will be real nice. They supposedly sound a lot like Mullard XF1's. Same production lot, X13X9C. They're pulls but they were fully tested and have > 80% life left in them so they should be good for a nice long time, especially with my limited use plan. Tube Depot has NOS versions for sale for $399.95 each.