Best tubes for Audio Note Cobra Integrated

Input tubes have the most effect on sonic signature, specifically the gain stage tube.

I don't own a Cobra but have an AN preamp and DAC. Have also owned a few tube amps.

The Cobra has some unusual preamp tubes. I’d leave the 6AU6 alone for the moment. You could try rolling the GE 5670 with Western Electric 396A.
When power tube time comes, try new Tung Sol EL34B or 7581A.

NOS Siemens RFT EL34's.  Expect to pay $400 - $500 for a quad.

First, Gold Lion KT77 to replace EL34. It improves the sound with more analytical way.

Then, replace GE5670 with WE396a / NE396a. It will becomes more musical. But you may found it is still not full enough. 

Finally, replace GE 6au6 with Mullard 6au6.

You won’t regret to own Cobra after all these………