I have yet to try other tubes in the TC-100 with exception to the MESA NOS SED =C= EL34s STR442. Those I actually liked better in the RA100 than the stock STR417 EL34s but those sound killer in the TC-50 and TC-100 due to the higher plate voltage. I would say that the STR440 would be your best bet.
As for the 6L6GC tube, the STR 440 is what is used in the official demo video and are the stock tubes used in the models that are intended for the 6L6 tube. JP-2C, Rectifier amps, Mark V, and the Lonestar 100W. Not sure what to recommend for the TC-100. :roll: I personally liked the STR425 in the TC-50 over the STR440 but that amp is a bit different than the TC-100 in its tonal output. So I had to find out for myself while writing this response, I figured why not give the STR440 (have a new quad of yellows I got for the Roadster) in the TC-100………
The result, :shock: well it is like this, clean channel will be clean, actually I did not notice a dramatic change to the clean channel, still has that Fender chime to it, nice punchy low end with that piano tone to it, top end was bright and pleasant. the drive mode, well that was quite different. Instead of having a plexi voice with the gain maxed out pushing the EL34s, it now has a Tweed voice, and it does not kill the reverb circuit like it does in the Roadster and Mark V, that was worth it. CH2, still on the thin side as it is with the EL34 but much better than I expected. The STR440 do have an extended midrange (or at least that is how they sound to me compared to other tubes like SED =C= 6L6GC, or Tung Sol 7581A. That had more of a character to the crunch channel on the Mark V, only better. That circuit is similar though as it uses two gain stages before the tone stack. Adjust the presence control a bit and now it is getting on the brink of the IIC+ voice of the Mark V, not the actual IIC+ mind you, tight mode was even better. That was unexpected. CH3 just blew my mind. :!: That I did not expect at all. :shock: This TC-100 is a Gem. What a hidden treasure to find. Hmm. I need to see how that compares to the JP-2C. The great thing about it, you do not need the 5BGEQ. Now I am not sure I will ever go back to the EL34s as the STR440 rock! No mud, tight bass response it is almost perfect. I still like the STR417 EL34s but the STR 440 6L6GC are definitely worth the coin. Go figure, I thought they sounded a bit old in the TC-50 or lacking, they were not bad mind you but just softer than I expected. TC-100 is definitely a different animal. OMG, I would recommend the STR440, Yellows are a good choice. I need to explore this a bit more.
Thanks for the question since you got my curiosity I had to try them. 8) :mrgreen: GET THE STR440! Holy crap, not sure why I waited so long to try them.