Newbee, hit's the note as you can change out the KT88 with the 6550, but you got to re-adjust the bias. The KT88 are probably better sounding than the 6550, but unfortunately they are a more fragile tube. If you can get a quartet of KT88s past 30 days you're probably all right, but unfortunatley, there does seem to be a high failure rate with the KT88 tube. I'm currently using the current issue of the Tung-Sol 6550, which seems to be good tubes, but true be told the Sovtek 6550 is the standard for realiability.
