what best internal cable in tube preamp?

Depends on what you are looking to achieve sonically. Please let us know the change or improvement you want. I can then give you good direction. 

I know Cardas and Duelund always tend to be the first that are voiced, but DH Labs makes incredible internal wiring for components and speakers. Their wiring includes copper, silver coated copper, or pure silver all in different gauges, as well as a variety of dielectrics to suit. They are also the OEM cable of choice for many component and speaker manufacturers. 

https://silversonic.com/products/hookup-wire/