Alot of modern KT88s are 6550 guts in a different bottle - 'specially the Sovtek stuff. That doesn't make them bad, they're just not KT88s.
6550s are rated for 42w max dissipation, however it all depends on the output transformer whether you get that sort of output.
If you put them in an amp that uses EL34s you'll probably get a little bit more volume out of them, but not almost double the volume (EL34s are rated for 25w). What you get is more headroom, extended bass end, clearer treble and the mids depends on the valve but plain-vanilla Sovtek 6550s are very crunchy in the mids whereas the Svetlana 6550s are much deeper in the bass and more 'controlled'.
My mate's JCM800 that I modded runs 6550s and he always buys EH 6550s as they sound good and we've had good reliability with them. Last set lasted two years of gigging abuse.
I swear by 6550s in JCM800s if you err towards the metal side of things.
:twisted:
