The E810F is an amazing tube. IMHO is not the best in all circuits, and depends on system.
The main "problem" of this tube, is that is extremely high transconductance tube, and very fast, and with our systems, is right on the edge of stability in terms of oscillation. Some samples of this tube can be great for some time and one day can start ringing. Nothing bad will happen to equipment just annoying and you need to change it.
For this reason alone, we do not use it in more equipment , BUT, we suggest users use it with the fine print and having always in mind that their tube one day might start singing.
By experience, only about 10-20% max of samples do this, and usually present this at very early days/hours.
Other than that, we are "hoarding" the hell of this tube
Yes on Kassandra, Ithaka and Ithaka minor is great with this tube. But is sometimes system depending. On Ithaka minor at the recent show, we had E280F for example.
Cheers
Stavros