I don't think you can lump all Groove Tubes together, this is a (partial) list of what they sell/used to sell:
- GT-EL34LS - Made in Slovakia
GT-EL34C - Made in China, cheap
GT-EL34R - Made in Russia, similar to EH-EL34 (rebadged)
GT-EL34R2 - Made in the other Russian factory, similar to Svetlana EL-34 (rebadged)
GT-EL34M - Made in China, cheap copies of the Mullard XF2
GT-6CA7GE - Made in ?, some US parts
they also do a KT77 that will fit most EL34 amps.
they do not make any tubes, although their 6CA7 is manufactured by third parties to their specs.
Electro Harmonix sells/brands multiple types as well (true EL34 and KT77)
But this is even more complicated as Sovtek, Electro Harmonix, Winged =C= and Svetlana are all trademarks/brands owned by New Sensor.
They sell Russian made Sovteks and Svetlanas as Electro Harmonics.
They also sell the same tubes as Svetlana, Mullard reissues and Winged =C=.
Winged =C= is used in the USA where the Svetlana trade name is not owned by Svetlana/New Sensor, but they are the same tubes.
You pay for the logo/brand on them. There is no magic.