I own a pair of VTL225 with KT90 tubes. I talked to VTL, they recommend me to upgrade to 6550 tubes since they are more available and cheaper. They said they must change the amps to accept 6550. Does anyone know if KT90 and 6550 tubes are interchangeable? Does 6550 sound better than KT90?? Where can I get good KT90 tubes at reasonable price?
