I had VTL retube my Deluxe 300's w/ KT90s when they were first available as they had my amps for upgrades anyway. I think that was 6 years ago and they still are sounding good and biasing out fine. There were some dramatic improvents and tradeoffs from the stock 6550A's, though I did not have the amps very long before the repair/retube. My recollections on the differences are that the KT90's are more detailed and tighter bass in my amps at the expense of being more analytical and less "warm" sounding. Be careful, as I do believe there are 2 incarnations of KT90's that are not interchangeable. My tubes are designed to idle @ 300mv, and there are some that idle at twice that. I was on the phone with VTL 2 weeks ago and they told me that they are now usng what they feel betters them all, the 6550C's. By the way, if anyone wants to make me an offer on 14ea 6550A's (red VTL stamped), they are for sale. Unfortunately the only way to determine what YOU like with your amplification is the expensive way - try them all )that are compatible with your cicuitry). Good luck. Alan Tempe, AZ
