I read with interest someone's synopsis of the differences between Tung Sol 6F8G black glass...
Flat Plates vs Round Plates:
Round plates have boosted bass and mid-bass which covers a little bit of the midrange. Flat plates are flat but with better defined treble and more air. They're just flat across whole frequency range have the same super sonic quality with Round Plates but better holography and air. Round plates have boosted bass and because of that, more body - but are not as transparent and clear.
Whilst I've not suss out such differences, I do note that the round plates I'm listening to now with 5998, have a very smooth and boosted bass presentation. Now this could be because of 5998 or the Eikon headphone I'm using.
Which leads me to note the differences between 5998 and 6 x 6BL7. 5998 has a more boosted bass, more body and air. 6 x 6BL7 is flatter but more defined and precise. Both have impressed me with the sound presented to my ears.
5 months on with Glenn's OTL amp, I'm still discovering the best possible tone that appeals to me with certain tube combinations. This combination of Tung Sol 6F8G black glass round plates and 5998 is my current favourite. Coupled with the fact that each time I power the amp on with these tubes, I get the ultra black background with no noise at all, it's enough to make me give it a 5 stars ratings.