Questions on replacing tubes... (Sovtek and Marchall 6100)

[kungfoo]

I have two questions:

I want to replace the stock tubes of my Sovtek Tube Midget 50H with new ones. Which ones would you recommend? Right now the amp sports 5881s
I'm thinking about Svetlana SV6L6GC...

Those seem to have a good reputation....

My friends 5881s in a Marshall 6100 30th anniversary-top are dying out. Which ones would you pick there? Would it be a cool thing to replace them with 6550s? Perhaps also by Svetlana?
Does Replacing 5881s with 6550 (KT88 I think) involve any modification on the amp, except bias-tweaking??