Because the AE-25 accepts such a large number of tube types without any problem and there's no effort involved in biasing I've accumulated an embarrassingly large number of output tube sets. The NOS Tung Sol 5881's are some of my favorites and I tend to alternate them with EL-34's, either NOS Tesla or Siemens/RFT. I've spent less time listening to the KT-88's and 6550's but they also sound very good in the amp.