The KT120 tubes will work great in your Premier Eights. They should be biased the same as a 6550. The sound of the KT120s is fuller without any sacrifice in dynamics and high frequency extension. I much prefer them to the 6550 winged C tubes (my favorite of the 6550s that I have tried). I would definitely make the switch the next time you need to re-tube. Just plug in the tubes, bias and enjoy. They will be operating at lower voltages than they are rated for, which should extend the life of the tube.