Those tubes you have are very special. Also not so many people have tried the 6F8/6C8 tubes and they are very good, in many cases better than the similar 6SN7. Did you ever try the Frankember? I have re-discovered it recently, and it gives the largest soundstage of all my tubes, it is almost ridiculous!
At 10 bucks per piece is quite a nice sounding tube. Bass can be a bit overwhelming with certain types of music though.
No, I have not tried the 7193 "Franken-tubes". I would need a 'Franken' adapter to give them a try. I'll see what the 6P5G'ssound like as I can use the same adapter as the 6J5 and 6P5 & 6L5. Some of the folks who used to be regulars here are now over at the Feliks Elise or Euforia sites and seem to enjoy these tubes in that amp as well as the Ember!
I got one of the Kameleon prototypes from the designer, relatively cheap with two filter modules included. The effect is remarkable in my opinion. People may think that I am trying to justify my investment (150 EUR with the two filter modules), but the truth is that it has a much stronger effect (for good!) than (changing) tubes can possibly provide. I cannot listen to my HD650 unfiltered anymore, there is something missing in the bass without the filter. The Garage 1217 version is admittedly a bit on the expensive side though.
I may send the folks at Garage 1217 an email and inquire about using the Kameleon with the Nighthawks.