I use my K3 with an Al-811 and it works just fine. I had the amp before the K3 so I already had my settings for load and plate recorded. Just use the settings in the 811 manual as a starting point and tune to what works best for you then record those settings. I have replaced the original tubes with Chinese 572B tubes. No more power but they are a more rugged tube so I just tune for max power out. With the 811s you will want to limit tune time but once there they should be fine.
73,
Bill NZ0T