Which brand 6F8G was the, "best"(in your opinion)? The choices(and flavors) are as varied as the 6SN7s.
Cary SLP-05 Tube Rolling
If you have this preamp and haven't tried swapping out some tubes you don't know what you are missing. I tried all the 6SN7s and found a couple that were very good and they were the metal base Sylvania & the Russian 6N8S 1578 metal based tubes labeled OTK.I recently sold 60+ various 6SN7 tubes that I had tried. I found that the 6F8G tubes were better.To put it simply there was more of everything: detail, bass, soundstage etc. To use these you need an adapter because they are also powered from the top. These adapters are available on EBAY as well as the tubes. Quality of the adapters is variable as I have more then one DOA. An easy trial would be to purchase a matched pair of tubes and get a couple adapters.To get the biggest bang for your buck place them in the third position from the left as you are looking at the preamp. You will find that compared to the top rated 6SN7 tubes the 6F8G's are relatively inexpensive but in my humble opinion sound better.