For me the 6f8G is a step-up from the average 12au7 and a side step (different flavor) compared to a good 12au7.
I'm using the Tung Sol round plate version and i like it a lot.
Overall the mids aren't as forward as with most 12au7's, bass is different (deeper?), soundstage is bigger.
Big downside is that mine are microphonic
.