As a side note, I also looked up the TT21/TT22; new to me.
Reports are that the innards are the same as KT88 as Kubusi intimated. Higher dissipation ratings mainly in the quoted applications. Folks reckon that one cannot do much more with them than with KT88 while the price is notably higher. (No criticism intended, Kubusi!) Yes, higher anode voltage possible, but then one ends with the predicament Steinhart landed in, regarding protection.
In general, for several reasons it becomes desirable to go to two valves in parallel rather than the super-power behemoths. I do the same with parallelled 6L6GCs as already mentioned elsewhere (e.g. ending with 70W equivalent dissipation possibility and wider spread of heat source at not much higher cost). I would also cite the use of two x EL84 in parallel rather than one 25W valve, as used in the Manley Stingray, there taking advantage of the sensitivity of EL84 for easy drive.
But going OT; sorry Karel! Thanks for your good work.