Tube upgrades for SF Line Preamp

I recently purchased Sonic Frontier line preamp and upgraded the standard 6922 tubes to valvo tubes. Like most audiophile, I am needlessly spending more money hoping to achieve that 1% increase in sound reproduction. Does anyone know what other tubes I can upgrade to in order to get me to that next level?
You could try the Amperex JAN (Joint Army Navy) 7308, white label. For my taste, they are my favorites. Note: Only two (2) are required.
I'd call Kevin at Upscale Audio. He has far more knowledge of tubes than most of us and he is a big SF fan so he will really steer you right. He had great info for my Rogue 66. Mike
Have you tried the mullards, direct from SF? Give them a call
Hi 3chihuahuas, I agree with Zenaudio call Kevin he will fix you up. He knows your pre-amp and he knows his tubes.
I had great results with the Mullard tubes in my SFL-1
Thanks fellow audiophiles..
It would be nice to hear if there are other sources than Kevin at Upscale since he refuses to sell Amperex 7308 tubes to those of us not resident in the USA. Does anyone know of another source?