Another thing to add is which Svetlanas? The original Svetlana company now has to call their tubes Winged C in the US and Canada, and the ones now called Svetlana, which being good tubes, are not the same or as good as the ones from the St. Petersburg Svetlana factory. The current Svetlanas are made is Saratov Russia by New Sensor, who also owns the Sovtek, EH, etc brand names.
KT 88's will make more power in many designs than 6550's, but I'm not really sure what your friend is talking about here. It sounds like he really doesn't know what he's talking about either.
Greg