Burn-in time for 6550c tubes ?

Hi:

I have just re-tubed my CJ Premier 8 amps with brand new Svetlana 6550c tubes, having old and rusty Sovtek 6550 tubes before.

I have just droped-in the new tubes, and biased the amp accordingly - sound thru the Premiers is now more linear but constrained.

Is this normal?
Do the new 6550c tubes need of a burn-in phase?
How long?

Please consider that all input drivers are quite new (less than 3 months).

Thanks for your valuable input.

Fernando