I find that the 6550 tubes, while having good dynamics and deep bass, are very thin and cold sounding. Improvements are the cousins to these tubes - KT88, KT90, KT91, etc. While a bit of power is sacrificed, they tend to be more musical. The only reason the 6550 tubes are used so much, is due to the wide avilability of them and the number of suppliers. There are better tubes that produce much better "musicalility" such as the 6C33s. The problem is there is only one source that I am aware of. These tubes are used in the better products, such as the Lamms and the BAT equipment.
