I have a 1974 Sunn Model T which has an excellent sound and came equipped with old American Tung-Sol 6550's. It's time to replace those babies and I need some good "sound" advice

I'm not in to "gritty" or "harsh" sound.
Which tubes would be best?
>Russian
Tung Sol?
>Sovtek?
>Electro Harmonix?
>Svetlana?
>SED
Or???