VTL 450 6550 or KT88 tubes

I've a pr of VTL 450'S has anyone tried these two types of tubes?
What do you think is the better of the two for this amp.
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I have the same amp and am using SED (Winged C) 6550's. I've been told by reliable sources that KT88's will give more extension on the highs, but not quite as nice in midrange. I haven't confirmed this. I also will soon be doing some 12AT7 tube rolling - which I hear is highly recommended. I'm using my amps to power the top end of Maggie 20.1's. What are you using yours for?
A VTL amp sounds great with a pair of Brimar 12AT7 black plate with the copper posts a NOS tube. I use them in a VTL 125 mono block amp and like it very much. I use EL 34 tubes in My amp but just tried KT77 tubes and like it a lot I have no experience with 6550 tubes or KT88 tubes try it you may like it ? Marc