@tksteingraber I have found that the National Union 6F8G round plates are very special tubes in my Supratek Syrah. I also have the NU 6SN7 round plates and the NU 6F8G sound better in every way. Also have Tung Sol VT-231 round plates (transparency kings), KenRad VT-231 staggered plates (bass kings), Sylvania T-plates, Baldwin flat plates, Sophia Electric, and Shuguang Treasure CV181-Z and the NU 6F8Gs are my “go to” tubes.
Keep in mind that the Supratek power supply’s rectifier tube paired with the output tubes has a significant influence on the sound (I like the Phillips 5R4G rectifier with NU 6F8G while the Mullard 5AR4/GZ34 Fat Base rectifier pairs better with the TS round plates IMO), so some mixing and matching of rectifiers is important.
Re: Raytheon 6F8G T-plates vs flat plates, the T-plates are much easier to find but the flat plates (scarce as hen’s teeth) are said by many to be significantly superior sonically.