Clipsal,
As Stiltskin says, only use "g" do not use metal cans(6F6). Both the 6F6g and the 6F6GT are identical tubes. The only difference is the shape of the glass. The 6F6gt has a straight side.
http://www.radiomuseum.org/tubes/tube_6f6gt.html
I have several of each type (Visseaux 6F6G; Tung-Sol 6F6g; Tung-Sol 6F6gt) and can hear no difference between the tung-sol 6F6g and the tung-sol 6F6GT. The Visseaux 6F6g, while maintaining the qualities of the tung-sol 6f6g (speed,resolution), is not as linear. They have a bit more high end extension. Now this difference can most likly be contributed to diferences in the 'house sound' between Tung-Sol and Visseaux.
I find the 'tung-sol sound' less fatiguing during long listening sessions. I also like the look of the tung-sol 6F6GT, not unlike tung-sol 6sn7gt on steriods!
Last night (with a 6106 and a pair of tung-sol 6sn7gt RPs) I tried a pair of tung-sol 6L6WGB from the 50's in the regulator position. Great big rich sound with some substance! No comparison to the sovtek 5881 or the sovtek kt66 which in comparison have a small and anemic presentation. Not sure if it's sovtek in particular but these new production tubes have lost the Mojo!
Enjoy the music!